Hydroxymethyl Linkers For Labeling Nucleotides

ABSTRACT

The invention provides methods and compositions, including, without limitation, algorithms, computer readable media, computer programs, apparatus, and systems for determining the identity of nucleic acids in nucleotide sequences using, for example, data obtained from sequencing by synthesis methods. The methods of the invention include correcting one or more phenomena that are encountered during nucleotide sequencing, such as using sequencing by synthesis methods. These phenomena include, without limitation, sequence lead, sequence lag, spectral crosstalk, and noise resulting from variations in illumination and/or filter responses.

FIELD OF THE INVENTION

The invention relates to methods, compositions, devices, systems andkits are described including, without limitation, reagents, mixtures,data processing steps, algorithms, computer readable media, and computerprograms, for determining the identity of nucleic acids in nucleotidesequences using, for example, data obtained from sequencing by synthesismethods.

BACKGROUND OF THE INVENTION

Over the past 25 years, the amount of DNA sequence information that hasbeen generated and deposited into Genbank has grown exponentially. Manyof the next-generation sequencing technologies use a form of sequencingby synthesis (SBS), wherein specially designed nucleotides and DNApolymerases are used to read the sequence of chip-bound, single-strandedDNA templates in a controlled manner. To attain high throughput, manymillions of such template spots are arrayed across a sequencing chip andtheir sequence is independently read out and recorded.

Devices, equations, and computer systems for forming and using arrays ofmaterial on a substrate for DNA sequencing are known (e.g., Ju et al.,U.S. Pat. No. 6,664,079; Pirrung et al., U.S. Pat. No. 5,143,854;Hubbell et al., U.S. Pat. No. 5,71,639; Lipshutz et al., U.S. Pat. Nos.6,957,149, 5,733,729, 6,066,454, 6,228,593 and 6,546,340; Chee et al.,U.S. Pat. No. 5,795,716; Domnisoru et al., U.S. Pat. No. 6,598,013;Schermer et al., U.S. Pat. No. 7,209,836; Gavrilov et al., U.S. Pat.Application No. 2007/0194249; Eltoukhy et al. In: IEEE InternationalConference on Acoustics, Speech and signal processing, (2006)2:1032-1035; Margulies et al. (2005) Nature 437:376-380; and Gerardo etal. (2008) Nucleic Acids Res. (2008) 36(4):e25). However, there is acontinued need for methods and compositions for increasing the fidelityof sequencing nucleic acid sequences.

SUMMARY OF THE INVENTION

The invention provides methods, compositions, devices, systems and kitsare described including, without limitation, reagents, mixtures, dataprocessing steps, algorithms, computer readable media, and computerprograms, for determining the identity of nucleic acids in nucleotidesequences using, for example, data obtained from sequencing by synthesismethods. The methods of the invention include reducing and/or correctingone or more phenomena that are encountered during nucleotide sequencing,such as using sequencing by synthesis methods. These phenomena include,without limitation, sequence lead, sequence lag, spectral crosstalk,light from neighboring spots, and noise resulting from variations inillumination and/or filter responses.

In one embodiment, the present invention contemplates a set of dataprocessing steps that may be used to analyze images of a hexagonal arrayof spots or beads on a surface. In one embodiment, the steps comprise a)field flattening and background subtraction, b) spot location in thearray, c) image sharpening, d) spot brightness determination, e)neighbor influence elimination, and f) color crosstalk elimination. Eachof these steps is described in more detail below. Of course, in oneembodiment, the present invention contemplates using a subset of thesesteps (in the same order or in a different order) as well as additionalprocessing steps. The result of the analysis may be used to makemeasurements of the output of four different fluorescent dyes for eachspot in the array. The methods described may also be generalized for arectangular or other shaped arrays rather than a hexagonal array.

In one embodiment, the invention provides a method for determining anidentity of a nucleic acid at an interrogation position in a nucleotidesequence from data acquired from one or more channels, comprising a)obtaining a data set for one or more probe intensities at one or morenucleic acid positions in the sequence, wherein each probe correspondsto a nucleic acid, b) determining the ratio contribution to probeintensity at the interrogation position from probe intensities at theinterrogation position and at one or both of i) at least one subsequentnucleic acid positions in the sequence, and ii) at least one precedingnucleic acid positions in the sequence, and c) applying the ratiocontribution to probe intensity to the data set to arrive at an identityfor a nucleic acid at the interrogation position in the nucleotidesequence. In a particular embodiment, the step of determining the ratiocontribution to probe intensity comprises measuring the rate (that is,the fraction of template molecules in an ensemble of identical templatemolecules) at which a lag, such as Gi, occurs at one or more nucleotideposition in the nucleotide sequence, such as at each nucleotide positionin the nucleotide sequence. In another embodiment, the step ofdetermining the ratio contribution to probe intensity comprisesmeasuring the rate (fraction) at which a lead, such as Di, occurs at oneor more nucleotide positions in the nucleotide sequence. In yet anotherembodiment, the method further comprises calling a nucleic acid at theinterrogation position in the nucleotide sequence. In a furtherembodiment, the method comprises repeating steps b) and c) to arrive atan identity for a nucleic acid at more than one interrogation positionin the nucleotide sequence.

While not intending to limit the invention's method to particular steps,in one embodiment, the method further comprises a) applying a sequencelead-lag compensation equation to determine the ratio contribution toprobe intensity from probe at i) the interrogation position, ii) eachposition preceding the interrogation position, and iii) each positionsubsequent to the interrogation position, and b) summing up the ratiocontribution to probe intensity. In an alternative embodiment, the stepof applying of the ratio contribution to probe intensity comprises a)comparing probe intensities from the one or more channels at theinterrogation position, b) selecting the highest probe intensity of thecompared probe intensities, and c) calling a nucleic acid, whichcorresponds to the selected probe, at the interrogation position.

It is not intended to limit the invention to a particular mathematicalformula. Nonetheless, in one embodiment, the method comprises applying asequence lead-lag compensation equation to the ratio contribution toprobe intensity at a plurality of positions in the sequence. In oneparticular embodiment, the sequence lead-lag compensation equation isdetermined by applying equation

$\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{MN}\end{bmatrix} = {K_{{Lead}/{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{AN}\end{bmatrix}}$

where

I_(M1) is a probe intensity measured at position 1 in the sequence,

I_(M2) is a probe intensity measured at position 2 in the sequence,

I_(MN) is a probe intensity measured at position N in the sequence,

I_(A1) is the actual probe intensity at position 1 in the sequence,

I_(A2) is the actual probe intensity at position 2 in the sequence,

I_(AN) is the actual probe intensity at position N in the sequence,

where

$K_{{Lead}/{Lag}} = \begin{bmatrix}R_{{{Lag}/{Lead}},1} & R_{{{+ 1}\; {Lead}},1} & R_{{{+ 2}{Lead}},1} & R_{{{+ 3}{Lead}},1} & \ldots & R_{{{+ {({N - 1})}}{Lead}},1} \\R_{{{- 1}{Lag}},2} & R_{{{Lag}/{Lead}},2} & R_{{{+ 1}{Lead}},2} & R_{{{+ 2}{Lead}},2} & \ldots & R_{{{+ {({N - 2})}}{Lead}},2} \\R_{{{- 2}{Lag}},2} & R_{{{- 1}{Lag}},3} & R_{{{Lag}.{Lead}},3} & R_{{{+ 1}{Lead}},3} & \ldots & R_{{{+ {({N - 3})}}{Lead}},3} \\R_{{{- 3}{Lag}},4} & R_{{{- 2}{Lag}},4} & R_{{{- 1}{Lag}},4} & R_{{{Lag}/{Lead}},4} & \ldots & R_{{{+ {({N - 4})}}{Lead}},4} \\\vdots & \vdots & \vdots & \vdots & \ddots & \vdots \\R_{{{- {({N - 1})}}{Lag}},N} & R_{{{- {({N - 2})}}{Lag}},N} & R_{{{- {({N - 3})}}{Lag}},N} & R_{{{- {({N - 4})}}{Lag}},N} & \ldots & R_{{{Lag}/{Lead}},N}\end{bmatrix}$

where

R_(Lag/Lead,1) is the ratio between reduced probe intensity for nucleicacid at position 1 to actual probe intensity at the nucleic acid atposition 1,

R_(+1Lead,1) is the ratio contribution to probe intensity at nucleicacid position 1 from probe at nucleic acid position 2,

R_(+Lead,1) is the ratio contribution to probe intensity at nucleic acidposition 1 from probe at nucleic acid position 3,

R_(+3Lead,1) is the ratio contribution to probe intensity at nucleicacid position 1 from probe at nucleic acid position 4,

R_(+(N-1)Lead,1) is the ratio contribution to probe intensity at nucleicacid position 1 from probe at nucleic acid position 1+(N−1),

R_(−Lag,2) is the ratio contribution to probe intensity at nucleic acidposition 2 from probe at nucleic acid position 1,

R_(Lag/Lead,2) is the ratio between reduced probe intensity for nucleicacid at position 2 to actual probe intensity at the nucleic acid atposition 2,

R_(+1Lead,2) is the ratio contribution to probe intensity at nucleicacid position 2 from probe at nucleic acid position 3,

R_(+2Lead,2) is the ratio contribution to probe intensity at nucleicacid position 2 from probe at nucleic acid position 4,

R_(+(N-2)Lead,2) is the ratio contribution to probe intensity at nucleicacid position 2 from probe at nucleic acid position 2+(N−2),

R_(−2Lag,3) is the ratio contribution to probe intensity at nucleic acidposition 3 from probe at nucleic acid position 1,

R_(−1Lag,3) is the ratio contribution to probe intensity at nucleic acidposition 3 from probe at nucleic acid position 2,

R_(Lag/Lead,3) is the ratio between reduced probe intensity for nucleicacid at position 3 to actual probe intensity at the nucleic acid atposition 3,

R_(+1Lead,3) is the ratio contribution to probe intensity at nucleicacid position 3 from probe at nucleic acid position 4,

R_(+(N-3)Lead,3) is the ratio contribution to probe intensity at nucleicacid position 3 from probe at nucleic acid position 3+(N−3),

R_(−3Lag,4) is the ratio contribution to probe intensity at nucleic acidposition 4 from probe at nucleic acid position 1,

R_(−2Lag,4) is the ratio contribution to probe intensity at nucleic acidposition 4 from probe at nucleic acid position 2,

R_(−1Lag,4) is the ratio contribution to probe intensity at nucleic acidposition 4 from probe at nucleic acid position 3,

R_(Lag/Lead,4) is the ratio between reduced probe intensity for nucleicacid at position 4 to actual probe intensity at the nucleic acid atposition 4,

R_(+(N-4)Lead,4) is the ratio contribution to probe intensity at nucleicacid position 4 from probe at nucleic acid position 4+(N−4),

R_(−(N-1)Lag,N) is the ratio contribution to probe intensity at nucleicacid position N from probe at nucleic acid position N−(N−1),

R_(−(N-2)Lag,N) is the ratio contribution to probe intensity at nucleicacid position N from probe at nucleic acid position N−(N−2),

R_(−(N-3)Lag,N) is the ratio contribution to probe intensity at nucleicacid position N from probe at nucleic acid position N−(N−3),

R_(−(N-3)Lag,N) is the ratio contribution to probe intensity at nucleicacid position N from probe at nucleic acid position N−(N−4), and

R_(Lag/Lead,N) is the ratio between reduced probe intensity for nucleicacid at position N to actual probe intensity at the nucleic acid atposition N.

In a further embodiment, the sequence lead-lag compensation equation isdetermined by applying equation

$\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{MN}\end{bmatrix} = {{K_{{Lead}/{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{AN}\end{bmatrix}}.}$

In a particular embodiment, the nucleic acid comprises a base selectedfrom the group of adenine (A), guanine (G), cytosine (C), thymine (T),and uracil (U), and the probe is detectable using any means such ascolor in the visible spectrum (e.g., fluorescence), radioactivity, andthe like.

While not intending to limit the invention's methods to particularsteps, in one embodiment, the methods further comprise field flatteningof background data for the data set. This may be accomplished by, forexample, a) obtaining a first data set for a plurality of pixelintensities of a first raw image of a probe at a first concentration ona solid support, wherein the first raw image is produced using a firstspectral filter for detecting a first probe, b) obtaining a second dataset for a plurality of pixel intensities of a second smoothed image ofthe probe uniformly spread on the solid support or other uniformlyradiating substrate, wherein the second smoothed image is produced usinga low pass filter, c) determining a field flattening intensity value fora plurality of pixels of the first raw image, and d) generating a thirdfield flattened image of the probe on the solid support using the fieldflattening intensity of the plurality of pixels, wherein the correlationof intensity of a plurality of pixels to their spatial location on thethird field flattened image is reduced compared to the intensity of aplurality of pixels at a corresponding location on the first raw image.In a particular embodiment, the background intensities are removed fromboth the first and second data sets so that the lowest intensity datapoint is at 0.

Although the field flattening methods are not intended to be limited toany particular equation, in one embodiment, the field flatteningintensity value of a pixel is determined by equation

F _(x,y) =R _(x,y) M _(x0,y0) /M _(x,y)

where

-   -   F_(x,y) is a field flattening intensity value,    -   R_(x,y) is the intensity of a pixel of the plurality of pixels        on the first raw image,    -   M_(x,y) is the intensity of a pixel of the plurality of pixels        on the second smoothed image at a corresponding spatial location        to the pixel on the first raw image, and    -   M_(x0,y0) is the intensity of a reference pixel on the second        smoothed image or is an arbitrary scale factor.        In one embodiment, the scale factor M_(x0,y0) may also include a        factor accounting for different exposure times or lighting        intensities. In another embodiment, such as where a camera        system has a proportional response to changes in exposure times        or lighting conditions, the following equation may be used

M _(x0,y0) =M ₀ E _((second image)) /E _((first image))

where E_((first image)) is the exposure or lighting level used duringmeasurement of the first image, E_((second image)) is the exposure orlighting level used for the second image and M₀ is an arbitraryconstant. In a further embodiment, the method further comprisesrepeating steps a) to d), using a second spectral filter for detecting asecond probe. In an alternative embodiment, the method further comprisesrepeating steps a) to d), using the probe at a second concentration onthe solid support. The solid support is exemplified, but not limited to,a microscope slide and silicon chip.

Also without limiting the invention's methods to particular steps, inone embodiment, the methods further comprise reducing spectral crosstalkin the one or more channels, by a) determining spectral crosstalkfactors for each of the one or more probes in its corresponding channelfrom one or more adjacent channels, b) applying the spectral crosstalkfactors to determine a spectral crosstalk matrix, and c) applying thespectral crosstalk matrix to the data set for the one or more probeintensities. In a particular embodiment, the step of reducing spectralcrosstalk comprises a) determining probe intensity for one or moreprobes from one or more channels, wherein each channel corresponds to aprobe, b) determining the ratios of the probe intensities in the one ormore channels to arrive at signature ratios for the probe intensity inthe channels, c) applying the signature ratios in a matrix equation, andd) inverting the matrix equation to arrive at an inverted matrix. In oneembodiment, the method further comprises e) applying the inverted matrixto data from the one or more channels.

While not intending to limit reducing spectral crosstalk to anyparticular equation, in one embodiment, the step of determining spectralcrosstalk matrix comprises using equation

$\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix} = {K\begin{bmatrix}A \\B \\C \\D\end{bmatrix}}$ where $K = {\begin{bmatrix}1 & R_{AB} & 0 & 0 \\R_{BA} & 1 & R_{BC} & 0 \\0 & R_{CB} & 1 & R_{CD} \\0 & 0 & R_{BC} & 1\end{bmatrix}.}$

and where

-   -   M_(A) is the observed intensity in the channel for probe A,    -   M_(B) is the observed intensity in the channel for probe B,    -   M_(C) is the observed intensity in the channel for probe C,    -   M_(D) is the observed intensity in the channel for probe D,    -   A is the actual probe intensity of probe A,    -   B is the actual probe intensity of probe B,    -   C is the actual probe intensity of probe C,    -   D is the actual probe intensity of probe D,    -   R_(AB) is the ratio between (a) the portion of intensity in the        channel for probe A that is contributed by probe B, and (b) the        actual probe intensity of probe B,    -   R_(BA) is the ratio between (a) the portion of intensity in the        channel for probe B that is contributed by probe A, and (b) the        actual probe intensity of probe A,    -   R_(BC) is the ratio between (a) the portion of intensity in the        channel for probe B that is contributed by probe C, and (b) the        actual probe intensity of probe C,    -   R_(CB) is the ratio between (a) the portion of intensity in a        channel for probe C that is contributed by probe B, and (b) the        actual probe intensity of probe B,    -   R_(CD) is the ratio between (a) the portion of intensity in a        channel for probe C that is contributed by probe D, and (b) the        actual probe intensity of probe D, and    -   R_(DC) is the ratio between (a) the portion of intensity in a        channel for probe D that is contributed by probe C, and (b) the        actual probe intensity of probe C.        The above equation is solved to determine spectral crosstalk        matrix K⁻¹ and an estimate of the actual intensities of the        probes (A, B, C and D) using equation

$\underset{\ldots}{\begin{bmatrix}A \\B \\C \\D\end{bmatrix}} = {K^{- 1}\underset{\ldots}{\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix}}}$

In an alternative embodiment, the equation is solved to determine and/orestimate for actual probe intensities A, B, C and D.

The invention further provides an algorithm for processing data fornucleic acids in a nucleotide sequence, wherein the data is acquiredfrom one or more channels, the algorithm comprising a) determining theratio contribution to probe intensity in the one or more channels forone or more interrogation positions, from probe intensities at theinterrogation position and at one or both of i) at least one subsequentnucleic acid positions in the sequence, and ii) at least one precedingnucleic acid positions in the sequence, b) processing data from the oneor more channels to correct for sequence lead and sequence lag, and c)reconstructing the data in the one or more channels. In one embodiment,the step of processing data comprises applying the ratio contribution toprobe intensity to determine, for the probe at the one or moreinterrogation positions, a sequence lead-lag compensation equation.Without limiting the invention to any particular equation, in oneembodiment, the sequence lead-lag compensation equation is determined byapplying equation

$\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{MN}\end{bmatrix} = {K_{{Lead}/{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{AN}\end{bmatrix}}$

where

I_(M1) is a probe intensity measured at position 1 in the sequence,

I_(M2) is a probe intensity measured at position 2 in the sequence,

I_(MN) is a probe intensity measured at position N in the sequence,

I_(A1) is the actual probe intensity at position 1 in the sequence,

I_(A2) is the actual probe intensity at position 2 in the sequence,

I_(AN) is the actual probe intensity at position N in the sequence,

In an alternative embodiment, the sequence lead-lag compensationequation is determined by applying equation

$K_{{Lead}/{Lag}} = \begin{bmatrix}R_{{{Lag}/{Lead}},1} & R_{{{+ 1}\; {Lead}},1} & R_{{{+ 2}{Lead}},1} & R_{{{+ 3}{Lead}},1} & \ldots & R_{{{+ {({N - 1})}}{Lead}},1} \\R_{{{- 1}{Lag}},2} & R_{{{Lag}/{Lead}},2} & R_{{{+ 1}{Lead}},2} & R_{{{+ 2}{Lead}},2} & \ldots & R_{{{+ {({N - 2})}}{Lead}},2} \\R_{{{- 2}{Lag}},2} & R_{{{- 1}{Lag}},3} & R_{{{Lag}.{Lead}},3} & R_{{{+ 1}{Lead}},3} & \ldots & R_{{{+ {({N - 3})}}{Lead}},3} \\R_{{{- 3}{Lag}},4} & R_{{{- 2}{Lag}},4} & R_{{{- 1}{Lag}},4} & R_{{{Lag}/{Lead}},4} & \ldots & R_{{{+ {({N - 4})}}{Lead}},4} \\\vdots & \vdots & \vdots & \vdots & \ddots & \vdots \\R_{{{- {({N - 1})}}{Lag}},N} & R_{{{- {({N - 2})}}{Lag}},N} & R_{{{- {({N - 3})}}{Lag}},N} & R_{{{- {({N - 4})}}{Lag}},N} & \ldots & R_{{{Lag}/{Lead}},N}\end{bmatrix}$

where

-   -   R_(Lag/Lead,1) is the ratio between reduced probe intensity for        nucleic acid at position 1 to actual probe intensity at the        nucleic acid at position 1,    -   R_(+1Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position 2,    -   R_(+2Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position 3,    -   R_(+3Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position 4,    -   R_(+(N-1)Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position        1+(N−1),    -   R_(−1Lag,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position 1,    -   R_(Lag/Lead,2) is the ratio between reduced probe intensity for        nucleic acid at position 2 to actual probe intensity at the        nucleic acid at position 2,    -   R_(+1Lead,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position 3,    -   R_(+2Lead,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position 4,    -   R_(+(N-2)Lead,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position        2+(N−2),    -   R_(−2Lag,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position 1,    -   R_(−1Lag,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position 2,    -   R_(Lag/Lead,3) is the ratio between reduced probe intensity for        nucleic acid at position 3 to actual probe intensity at the        nucleic acid at position 3,    -   R_(+1Lead,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position 4,    -   R_(+(N-3)Lead,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position        3+(N−3),    -   R_(3Lag,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position 1,    -   R_(−2Lag,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position 2,    -   R_(−1Lag,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position 3,    -   R_(Lag/Lead,4) is the ratio between reduced probe intensity for        nucleic acid at position 4 to actual probe intensity at the        nucleic acid at position 4,    -   R_(+(N-4)Lead,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position        4+(N−4),    -   R_(−(N-1)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−1),    -   R_(−(N-2)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−2),    -   R_(−(N-3)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−3),    -   R_(−(N-4)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−4), and    -   R_(Lag/Lead,N) is the ratio between reduced probe intensity for        nucleic acid at position N to actual probe intensity at the        nucleic acid at position N.        In another alternative embodiment, the sequence lead-lag        compensation equation is determined by applying equation

$\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{MN}\end{bmatrix} = {K_{{Lead}/{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{AN}\end{bmatrix}}$

While not necessary, it may be desirable to also include fieldflattening of background data in the algorithm and/or reducing spectralcrosstalk between the data comprised in a plurality of channels.Dephasing correction (i.e., correction for lead-lag effects), fieldflattening and spectral crosstalk correction may be carried out in anyorder. Thus, in one embodiment, the field flattening is carried outbefore spectral crosstalk correction. In an alternative embodiment,spectral crosstalk correction is carried out before dephasingcorrection.

The invention also provides a computer readable medium containing acomputer program for performing one or more of the method stepsdisclosed herein.

Also provided by the invention is a computer program product forprocessing data for nucleic acids in a nucleotide sequence to determinean identity of a nucleic acid at an interrogation position in thenucleotide sequence, the computer program product comprising a) computercode that inputs data from one or more channels for one or more probeintensities, wherein each channel corresponds to a probe, and each probecorresponds to a nucleic acid, b) computer code that applies to theinput data a sequence lead-lag compensation equation to correct forsequence lead and sequence lag, c) computer code that compares probeintensities in the one or more channels that have been corrected forsequence lead and sequence lag, d) computer code that determines thehighest probe intensity of the compared probe intensities, and e)computer code that identifies a nucleic acid at the interrogationposition according to the highest probe intensity. Optionally, thecomputer program product may further comprise computer code that appliesfield flattening of background data and/or that reduces spectralcrosstalk between data comprised in the one or more channels.

The invention also provides an apparatus that processes data for nucleicacids in a nucleotide sequence to determine an identity of a nucleicacid at an interrogation position in the nucleotide sequence, theapparatus comprising a) means for inputting data from one or morechannels for one or more probe intensities, wherein each channelcorresponds to a probe, and each probe corresponds to a nucleic acid, b)means for applying to the input data a sequence lead-lag compensationequation to correct for sequence lead and sequence lag, c) means forcomparing probe intensities in the one or more channels that have beencorrected for sequence lead and sequence lag, d) means for determiningthe highest probe intensity of the compared probe intensities, and e)means for identifying a nucleic acid at the interrogation positionaccording to the highest probe intensity. Though not necessary, it maybe desirable to also include means for applying field flattening ofbackground data and/or for reducing spectral crosstalk between datacomprised in the one or more channels.

Additionally provided herein is a system for processing data todetermine an identity of a nucleic acid at an interrogation position inthe nucleotide sequence, the system comprising a) a processor, and b) acomputer readable medium readable by the processor, the computerreadable medium storing a computer program that comprises i) code thatreceives as input a plurality of probe intensities at various positionsin a nucleotide sequence, ii) code that applies to the input data asequence lead-lag compensation equation to correct for sequence lead andsequence lag, and iii) code that identifies a nucleic acid at one ormore interrogation position according to the corrected data. While notnecessary, it may be desirable to additionally include in the computerreadable medium code that applies field flattening of background dataand/or that reduces spectral crosstalk between data comprised in the oneor more channels.

The invention also provides a method for field flattening an image of aprobe on a solid support, comprising a) obtaining a first data set for aplurality of pixel intensities of a first raw image of a probe at afirst concentration on a solid support, wherein the first raw image isproduced using a first spectral filter for detecting a first probe, b)obtaining a second data set for a plurality of pixel intensities of asecond smoothed image of the probe on the solid support, wherein thesecond smoothed image is produced using a low-pass filter, c)determining a field flattening intensity value for a plurality of pixelsof the first raw image, and d) generating a third field flattened imageof the probe on the solid support using the field flattening intensityof the plurality of pixels, wherein the correlation of intensity of aplurality of pixels to their spatial location on the third fieldflattened image is reduced compared to the intensity of a plurality ofpixels at a corresponding location on the first raw image. Withoutintending to limit the invention to any particular equation, in oneembodiment, the field flattening intensity value of a pixel isdetermined by equation

F _(x,y) =R _(x,y) M _(x0,y0) /M _(x,y)

where

-   -   F_(x,y) is a field flattening intensity value,    -   R_(x,y) is the intensity of a pixel of the plurality of pixels        on the first raw image,    -   M_(x,y) is the intensity of a pixel of the plurality of pixels        on the second smoothed image at a corresponding spatial location        to the pixel on the first raw image, and

M_(x0,y0) is the intensity of a reference pixel on the second smoothedimage, or is any other scale factor of interest.

In one embodiment, it may be desirable to repeat steps a) to d), using asecond spectral filter for detecting a second probe. Alternatively, orin addition, it may be desirable to repeat steps a) to d), using theprobe at a second concentration on the solid support. In one embodiment,the probe is fluorescent and corresponds to a nucleic acid thatcomprises a base selected from the group of adenine (A), guanine (G),cytosine (C), thymine (T), and uracil (U). The solid support maycomprise a microscope slide, silicon chip, and the like.

The invention also provides a method for reducing spectral crosstalk inone or more channels that deliver data for determining the identity of anucleic acid at an interrogation position in a nucleotide sequence,comprising a) obtaining a data set for one or more probe intensities atone or more nucleic acid positions in the sequence, wherein each probecorresponds to a nucleic acid, b) determining spectral crosstalk factorsfor each of the one or more probes in its corresponding channel from oneor more adjacent channels, c) applying the spectral crosstalk factors todetermine a spectral crosstalk matrix, and d) applying the spectralcrosstalk matrix to the data set to arrive at an identity for a nucleicacid at the interrogation position in the nucleotide sequence. In oneembodiment, the step of determining spectral crosstalk factors comprisesdetermining a ratio between (a) the portion of probe intensity in afirst channel of a first probe that is contributed by a second probe ina second channel adjacent to the first channel, and (b) the actual probeintensity of the second probe in the second channel. In a particularembodiment, the method further comprises determining the ratio between(a) the portion of probe intensity in the first channel of the firstprobe that is contributed by a third probe in a third channel adjacentto the first channel, and (b) the actual probe intensity of the thirdprobe in the third channel. Without limiting the type of equation used,in one embodiment, the step of determining spectral crosstalk matrixcomprises using equation

$\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix} = {K\begin{bmatrix}A \\B \\C \\D\end{bmatrix}}$ where $K = {\begin{bmatrix}1 & R_{AB} & 0 & 0 \\R_{BA} & 1 & R_{BC} & 0 \\0 & R_{CB} & 1 & R_{CD} \\0 & 0 & R_{BC} & 1\end{bmatrix}.}$

and where

-   -   M_(A) is the observed probe intensity of probe A,    -   M_(B) is the observed probe intensity of probe B,    -   M_(C) is the observed probe intensity of probe C,    -   M_(D) is the observed probe intensity of probe D,    -   A is the actual probe intensity of probe A,    -   B is the actual probe intensity of probe B,    -   C is the actual probe intensity of probe C,    -   D is the actual probe intensity of probe D,    -   R_(AB) is the ratio between (a) the portion of intensity in the        channel for probe A that is contributed by probe B, and (b) the        actual probe intensity of probe B,    -   R_(BA) is the ratio between (a) the portion of intensity in the        channel for probe B that is contributed by probe A, and (b) the        actual probe intensity of probe A,    -   R_(BC) is the ratio between (a) the portion of intensity in the        channel for probe B that is contributed by probe C, and (b) the        actual probe intensity of probe C,    -   R_(CB) is the ratio between (a) the portion of intensity in a        channel for probe C that is contributed by probe B, and (b) the        actual probe intensity of probe B,    -   R_(CD) is the ratio between (a) the portion of intensity in a        channel for probe C that is contributed by probe D, and (b) the        actual probe intensity of probe D, and    -   R_(DC) is the ratio between (a) the portion of intensity in a        channel for probe D that is contributed by probe C, and (b) the        actual probe intensity of probe C.        In a further embodiment, the equation is solved to determine        spectral crosstalk matrix K⁻¹ and an estimate of the actual        intensity or probes (A, B, C and D) using equation

$\underset{\ldots}{\begin{bmatrix}A \\B \\C \\D\end{bmatrix}} = {K^{- 1}\underset{\ldots}{\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix}}}$

In a particular embodiment, the order of the data correction methodsdescribed herein is 1) field flattening, 2) color crosstalk correctionand 3) dephasing correction. When field flattening precedes colorcrosstalk correction, then the same crosstalk parameters may be used forthe entire image. When color crosstalk correction precedes dephasingcorrection, the dephasing correction will be more accurate as theintensity data from the different channels will more precisely representactual probe intensities.

As noted above, the present invention contemplates reducing some ofthese phenomenon that make accurate base calling difficult. One problemaddressed in one embodiment of the present invention is the problemcreated by using a cleaving agent. In one embodiment, a cleaving agentscavenger is employed to address leftover cleaving agent which mightprematurely cleave in the next cycle. Thus, the present inventioncontemplates in one embodiment a method of incorporating labelednucleotides into nucleic acid, comprising: a) providing a plurality ofnucleic acid template molecules, a polymerase, a cleaving agent, acleaving agent scavenger, and a plurality of nucleotide analogueswherein each nucleotide analogue is labeled with a unique label andcontains a removable chemical moiety capping the 3′-OH group; b)incorporating a first nucleotide analogue with said polymerase; c)detecting the label of the incorporated nucleotide analogue; d) removingthe chemical moiety of the incorporated nucleotide analogue capping the3′-OH group with said cleaving agent; and f) incorporating a secondnucleotide analogue in the presence of said cleaving agent scavenger.With regard to step f), the scavenger can, by way of example, be putinto the solution used to incorporate nucleotides in the next round(indeed, in one embodiment, the present invention contemplatescompositions comprising 1) the scavenger(s) and one or more labeled orunlabeled nucleotides, 2) the scavenger(s) and polymerase, 3) thescavenger(s) and one or more nucleotides with or without 3-OH cappinggroups). Alternatively, the scavenger can be in a separate solution thatis used prior to the incorporation solution (with residual scavengerpresent at the time of incorporation). In one embodiment, the presentinvention contemplates wash steps after step b) and after step d).

It is not intended that the present invention be limited by the natureof the chemistry of the removable chemical moiety. A variety ofchemistries are contemplated (and described below in more detail). Inone embodiment, said removable chemical moiety comprises a disulfidebond. In another embodiment, said removable chemical moiety comprises anazido group (e.g. an azidomethyl ether). It is preferred that saidmoiety capping the 3′-OH is not a fluorescent moiety.

It is also not intended that the present invention be limited by thenature of the cleaving agent. In the case of azido-group-containingnucleotides (e.g. 3′-O-azidomethyl ether nucleotides), several types ofcleaving agents can be used. In principle, any reducing agent capable ofconverting the azido group into an amine is suitable for this purpose.The amine undergoes spontaneous conversion to hydroxyl group to enablenext nucleotide incorporation. Examples of cleaving agents include: a)Catalytic hydrogenation over PtO2 or Pd/C; b) Reduction with LiAlH4,HCO₂NH₄-10% Pd/C, NaBH₄/CoCl₂.6H₂O, Zn/NH₄Cl, Fe/NH₄Cl; and c) Reductionwith phosphines; e.g. tri-n-butyl-phosphine, triphenyl phosphine and itssulfonated versions (i.e., tris(3-sulfophenyl)-phosphine, TPPTS), andtri(carboxyethyl)phosphine (TCEP) and its salts. Most preferred cleavingreagents are soluble in water and are highly selective reducing agents.Water soluble phosphines are particularly preferred. In one embodiment,said cleaving agent is a phosphine Tris(2-carboxy-ethyl)phosphine.

It is also not intended that the present invention be limited by thenature of the cleaving agent scavenger. A variety of chemistries arecontemplated (and are described below and in the figures) and more thanone type of chemistry can be used together (e.g. two differentscavengers). In a preferred embodiment, said cleaving agent scavengerdoes not contain a nucleic acid base. In one embodiment, said cleavingagent scavenger comprises a disulfide bond (e.g. cystamine or one of theother disulfide-containing compounds shown in FIG. 37). Cystamine isalso known as 2,2′-Dithiobisethanamine, 2-Aminoethyl disulfide, orDecarboxycystine, and is available commercially from Sigma-Aldrich.Alternatively, the present invention contemplates in one embodiment thatsaid cleaving agent scavenger comprises an azido group (e.g. anazidomethyl group, an azidoethyl ether group, etc.). In a preferredembodiment, said scavenger is 11-Azido-3,6,9-trioxaundecan-1-amine(which is also known as: 1-Amino-11-azido-3,6,9-trioxaundecane,2-{2-[2-(2-Azidoethoxy)ethoxy]ethoxy}ethylamine, orO-(2-Aminoethyl)-O′-(2-azidoethyl)-diethylene glycol, and which isavailable commercially from Sigma-Aldrich).

It is not intended that the present invention be limited by where thefirst and second nucleotides are incorporated. In one embodiment, theyare incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

In some embodiments, two cites of cleavage are contemplated, i.e.cleavage occurs at two locations on the nucleotide analogue. Thus, inone embodiment, the present invention contemplates a method ofincorporating labeled nucleotides into nucleic acid, comprising: a)providing a plurality of nucleic acid template molecules, a polymerase,a cleaving agent, a cleaving agent scavenger, and a plurality ofnucleotide analogues selected from the group consisting of cytosine,thymine, deaza-adenine and deaza-guanine, wherein each nucleotideanalogue comprises a unique label attached through a cleavable linker toa 5-position of cytosine or thymine or to a 7-position of deaza-adenineor deaza-guanine, and wherein each nucleotide analogue contains aremovable chemical moiety capping the 3′-OH group; b) incorporating afirst nucleotide analogue with said polymerase; c) detecting the labelof the incorporated nucleotide analogue; d) removing the chemical moietyof the incorporated nucleotide analogue capping the 3′-OH group andcleaving the cleavable linker with said cleaving agent; and e)incorporating a second nucleotide analogue in the presence of saidcleaving agent scavenger.

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

Again, it is not intended that the present invention be limited by thenature of the chemistry of the removable chemical moiety. A variety ofchemistries are contemplated (and described below in more detail) andthe chemistry need not be the same chemistry as used in the cleavablelinker attaching the label. In one embodiment, said removable chemicalmoiety comprises a disulfide bond. In another embodiment, said removablechemical moiety comprises an azido group (e.g. an azidomethyl ether). Itis preferred that said moiety capping the 3′-OH is not a fluorescentmoiety.

Similarly, a variety of chemistries are contemplated for the cleavablelinker attaching the label to the nucleotide analogue (and these aredescribed in more detail below). In one embodiment, said cleavablelinker comprises a disulfide bond. As noted above, the present inventioncontemplates embodiments wherein the chemistries for the cleavage at thetwo cites is the same, as well as embodiments where it is different. Forexample, in one embodiment, said removable chemical moiety comprises anazido group (e.g. an azidomethyl ether) and said cleavable linker (whichattaches the label) comprises a disulfide bond. In another embodiment,this is reversed (the cleavable linker comprises an azido group and theremovable chemical moiety comprises a disulfide bond.

Again, it is also not intended that the present invention be limited bythe nature of the cleaving agent. However, in one embodiment, saidcleaving agent is a phosphine (e.g. Tris(2-carboxy-ethyl)phosphine).Again, a variety of cleaving agent scavengers are contemplated(discussed above). In a preferred embodiment, said cleaving agentscavenger does not contain a nucleic acid base.

In one embodiment, the present invention contemplates incorporatingnucleotides having only one location for cleavage (e.g. the cleavablelinker attaching the label). Thus, in one embodiment, the presentinvention contemplates a method of incorporating labeled nucleotidesinto nucleic acid, comprising: a) providing a plurality of nucleic acidtemplate molecules, a polymerase, a cleaving agent, a cleaving agentscavenger, and a plurality of nucleotide analogues wherein eachnucleotide analogue is labeled with a unique label, said label attachedby a cleavable linker; b) incorporating a first nucleotide analogue withsaid polymerase; c) detecting the label of the incorporated nucleotideanalogue; d) removing the label of the incorporated nucleotide analogueby cleaving the cleavable linker with said cleaving agent; and e)incorporating a second nucleotide analogue in the presence of saidcleaving agent scavenger.

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

Again, a variety of chemistries are contemplated for the cleavablelinker (e.g. wherein said cleavable linker comprises a disulfide bond,azido group, or some other chemical group). However, in a preferredembodiment, the chemistry of the cleavable linker dictates the chemistryof the scavenger (e.g. wherein said cleaving agent scavenger comprises adisulfide bond, it is preferred that the scavenger also comprise adisulfide bond, such as where said scavenger is cystamine or othersimilar compound).

In one embodiment, the present invention contemplates carrying outnucleotide incorporation in a device, including automated devices.Solutions comprising various combinations of biomolecules arecontemplated; such solutions can be, in one embodiment, conveniently bestored in reservoirs which are in fluid communication with a reactionchamber (e.g. flow cells, microchannels, etc.). A series of steps can becarried out to introduce these solutions (and the reagents they contain)into the reaction chamber (e.g. by valving) to carry out thereaction(s). Thus, in one embodiment, the present invention contemplatesa method of incorporating labeled nucleotides into nucleic acid,comprising: a) providing i) a reaction chamber (e.g. a flow cell)comprising plurality of nucleic acid template molecules bound to a solidsupport, ii) a first solution comprising polymerase and a plurality ofnucleotide analogues wherein each nucleotide analogue is labeled with aunique label and contains a removable chemical moiety capping the 3′-OHgroup, iii) a second solution comprising a cleaving agent, and iv) acleaving agent scavenger; b) introducing said first solution into saidreaction chamber under conditions wherein a first nucleotide analogue isincorporated by said polymerase; c) detecting the label of theincorporated nucleotide analogue; d) introducing said second solutioninto said reaction change under conditions such that the chemical moietyof the incorporated nucleotide analogue capping the 3′-OH group isremoved by said cleaving agent; and e) introducing said cleaving agentscavenger into said reaction chamber.

It is not intended that the present invention be limited by the way inwhich the cleaving agent scavenger is stored or introduced into thereaction chamber. In one embodiment, said cleaving agent scavenger is ina third solution and said scavenger is introduced into said reactionchamber in step e) by introducing said third solution. In anotherembodiment, the above-indicated method further comprises the step f)re-introducing said first solution into said reaction chamber underconditions such that a second nucleotide analogue is incorporated bysaid polymerase (and this first solution may contain the scavenger ifdesired). In another embodiment, separate steps [i.e. step e) and stepf)] are not required; rather, a single step is contemplated wherein saidcleaving agent scavenger is in said first solution and said introducingof step e) comprises introducing said first solution comprising saidscavenger (in this embodiment, a second nucleotide analogue isincorporated in the presence of said cleaving agent scavenger). In someembodiments, additional wash steps are employed to remove reagentsbetween steps [e.g. wash steps after step b), and step d)], although theusefulness of the scavenger has been discovered empirically, sinceresidual cleaving agent is difficult to remove with a practical numberof washes (discussed more below).

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

Again, it is not intended that the present invention be limited by thenature of the chemical moiety capping the 3′-OH on the nucleotideanalogue. In one embodiment, said removable chemical moiety comprises adisulfide bond. In one embodiment, said removable chemical moietycomprises an azido group (e.g. an azidomethyl ether). It is preferredthat said moiety capping the 3′-OH is not a fluorescent moiety.

Again, it is also not intended that the present invention be limited bythe nature of the cleaving agent. However, in one embodiment, saidcleaving agent is a phosphine (e.g. Tris(2-carboxy-ethyl)phosphine).Again, a variety of cleaving agent scavengers are contemplated(discussed above). In a preferred embodiment, said cleaving agentscavenger does not contain a nucleic acid base.

In some embodiments, the reaction in the device is directed at cleavageat two locations on the nucleotide analogue(s). Thus, in one embodiment,the present invention contemplates a method of incorporating labelednucleotides into nucleic acid, comprising: a) providing i) a reactionchamber comprising plurality of nucleic acid template molecules bound toa solid support, ii) a first solution comprising polymerase and aplurality of nucleotide analogues selected from the group consisting ofcytosine, thymine, deaza-adenine and deaza-guanine, wherein eachnucleotide analogue comprises a unique label attached through acleavable linker to a 5-position of cytosine or thymine or to a7-position of deaza-adenine or deaza-guanine, and wherein eachnucleotide analogue is labeled with a unique label and contains aremovable chemical moiety capping the 3′-OH group, iii) a secondsolution comprising a cleaving agent, and iv) a cleaving agentscavenger; b) introducing said first solution into said reaction chamberunder conditions wherein a first nucleotide analogue is incorporated bysaid polymerase; c) detecting the label of the incorporated nucleotideanalogue; d) introducing said second solution into said reaction changeunder conditions such that the chemical moiety of the incorporatednucleotide analogue capping the 3′-OH group is removed and saidcleavable linker is cleaved by said cleaving agent; and e) introducingsaid cleaving agent scavenger into said reaction chamber (e.g. flow cellor the like).

Again, it is not intended that the present invention be limited by theway in which the cleaving agent scavenger is stored or introduced intothe reaction chamber. In one embodiment, said cleaving agent scavengeris in a third solution and said scavenger is introduced into saidreaction chamber in step e) by introducing said third solution. Inanother embodiment, the above-indicated method further comprises thestep f) re-introducing said first solution into said reaction chamberunder conditions such that a second nucleotide analogue is incorporatedby said polymerase (and this first solution may contain the scavenger ifdesired). In another embodiment, separate steps [i.e. step e) and stepf)] are not required; rather, a single step is contemplated wherein saidcleaving agent scavenger is in said first solution and said introducingof step e) comprises introducing said first solution comprising saidscavenger (in this embodiment, a second nucleotide analogue isincorporated in the presence of said cleaving agent scavenger). In someembodiments, additional wash steps are employed to remove reagentsbetween steps [e.g. wash steps after step b), and step

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

Again, it is not intended that the present invention be limited by thenature of the chemical moiety capping the 3′-OH on the nucleotideanalogue. In one embodiment, said removable chemical moiety comprises adisulfide bond. In one embodiment, said removable chemical moietycomprises an azido group (e.g. an azidomethyl ether). It is preferredthat said moiety capping the 3′-OH is not a fluorescent moiety.

Again, the chemistry of the cleavable linker (which attaches the label)may be the same or different vis-à-vis the removable chemical cappingmoiety. Thus, in one embodiment, the linker and the capping groupcomprise a disulfide bond. Yet, in another embodiment, said removablechemical moiety comprises an azido group and said cleavable linkercomprises a disulfide bond (or the reverse, i.e. the capping groupcomprises a disulfide bond and the cleavable linker comprises an azidogroup).

Again, it is also not intended that the present invention be limited bythe nature of the cleaving agent. However, in one embodiment, saidcleaving agent is a phosphine (e.g. Tris(2-carboxy-ethyl)phosphine).Again, a variety of cleaving agent scavengers are contemplated(discussed above). In a preferred embodiment, said cleaving agentscavenger does not contain a nucleic acid base.

In some embodiments, the present invention contemplates a reaction inthe device wherein only a single cite of cleavage on the nucleotideanalogue is targeted (e.g. a cleavable linker attaching the label).Thus, in one embodiment, the present invention contemplates a method ofincorporating labeled nucleotides into nucleic acid, comprising: a)providing i) a reaction chamber comprising plurality of nucleic acidtemplate molecules bound to a solid support, ii) a first solutioncomprising polymerase and a plurality of nucleotide analogues whereineach nucleotide analogue is labeled with a unique label, said labelattached via a cleavable linker, iii) a second solution comprising acleaving agent, and iv) a cleaving agent scavenger; b) introducing saidfirst solution into said reaction chamber under conditions wherein afirst nucleotide analogue is incorporated by said polymerase; c)detecting the label of the incorporated nucleotide analogue; d)introducing said second solution into said reaction change underconditions such that the label of the incorporated nucleotide analogueis removed by cleaving said cleavable linker with said cleaving agent;and e) introducing said cleaving agent scavenger into said reactionchamber.

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

A variety of chemistries for the cleavable linker are contemplated. Inone embodiment, said cleavable linker comprises a disulfide bond.

In one embodiment, the chemistry used in the cleavable linker controlsthe chemistry of the scavenger. For example, in one embodiment, wherethe linker comprises a disulfide bond, said cleaving agent scavengercomprises a disulfide bond. In one embodiment, where the linkercomprises an azido group, said cleaving agent scavenger comprises anazido group. In a preferred embodiment, said cleaving agent scavengerdoes not contain a nucleic acid base.

The present invention contemplates methods, kits, devices, systems andcompositions. In one embodiment, the present invention contemplates acomposition comprising cleaving agent scavenger and one or morenucleotide analogues (unlabeled or labeled as herein described). In oneembodiment, said composition further comprises polymerase. In oneembodiment, the present invention contemplates a composition comprisingcleaving agent scavenger and polymerase, and (optionally) one or morenucleotide analogues (unlabeled or labeled as herein described).

In one embodiment, the present invention contemplates a reaction chamber(e.g. a flow cell, flow channels, etc.) comprising a solution, saidsolution comprising cleaving agent scavenger and one or more nucleotideanalogues (labeled or unlabeled as herein described). In one embodiment,said solution further comprises polymerase. In one embodiment, saidsolution comprises cleaving agent scavenger and polymerase, and(optionally) one or more nucleotide analogues (unlabeled or labeled asherein described).

In one embodiment, the present invention contemplates kits, said kitscomprising a solution comprising cleaving agent scavenger and one ormore nucleotide analogues (labeled or unlabeled as herein described) and(optionally) polymerase. Alternatively, said kits comprise a solutioncomprising cleaving agent scavenger and polymerase, and (optionally) oneor more nucleotide analogues (unlabeled or labeled as herein described).Preferrably, such kits also provide instructions for carrying outincorporation reactions, as well as wash buffers and the like.

In one embodiment, the present invention contemplates a systemcomprising reservoirs in fluid communication with a reaction chamber, atleast one of said reservoirs comprising a solution comprising cleavingagent scavenger and one or more nucleotide analogues (labeled orunlabeled as herein described) and (optionally) polymerase.Alternatively, at least one of said reservoirs comprises a solutioncomprising cleaving agent scavenger and polymerase, and (optionally) oneor more nucleotide analogues (unlabeled or labeled as herein described).Preferrably, such solutions can be introduced by automated means (e.g.valving).

As described herein, the present invention contemplates embodimentswherein nucleotides used in extension reactions contain linkers, spacersand chemical groups. The presence of these spacers and groups may affectthe ability of the sequencing polymerases to incorporate the subsequentnucleotide. The present invention contemplates a number of ways tominimize or eliminate this undesirable effect, including but not limitedto: a) reducing the amount of labeled nucleotides incorporated in thetemplate; b) reducing the size of the spacer arm or eliminate itcompletely by carefully designing nucleotide analogs; and c) change thereactivity of the spacer arm groups or their charge by performing achemical “capping” step, where specific reagent is added to react onlywith groups on the spacer arm.

Reducing the amount of labeled nucleotides that are incorporated can beaccomplished by reducing the concentration of labeled nucleotides in theextension solution, and/or by mixing labeled nucleotides (reversibleterminators) with non-labeled reversibly terminating nucleotides (e.g.where the non-labeled nucleotides are employed in ratios between 1:1 and1000:1 relative to the labeled nucleotides, but more preferably inratios between 10:1 and 100:1). In contrast to labeled nucleotides,non-labeled reversible terminator nucleotides after cleavage convert tonative nucleotide (and therefore do not present problems forpolymerases). Thus, in one embodiment, the present inventioncontemplates a composition comprising i) a first plurality of nucleotideanalogues wherein each nucleotide analogue is labeled with a uniquelabel and contains a removable chemical moiety capping the 3′-OH group;and ii) a second plurality of nucleotide analogues wherein eachnucleotide analogue is unlabeled and contains a removable chemicalmoiety capping the 3′-OH group. In one embodiment, the compositionfurther comprises polymerase. In a preferred embodiment, said nucleotideanalogues are in solution. In one embodiment, the second plurality ofnucleotide analogues is present in said solution at a high concentrationthan said first plurality of nucleotide analogues. In one embodiment,said second plurality of nucleotide analogues is present at aconcentration between 1 uM and 100 uM. In one embodiment, said firstplurality of nucleotide analogues is present at a concentration between1 nM and 1 uM.

It is not intended that the composition be limited by the number ornature of nucleotide analogues in said composition. However, in apreferred embodiment, said first plurality of nucleotide analoguescomprises four different nucleotide analogues (for example, in oneembodiment, the four nucletodes are either (i) aA, aC, aG, and aT, or(ii) aA, aC, aG, and aU). In a preferred embodiment, said secondplurality of (unlabeled) nucleotide analogues comprises four differentnucleotide analogues (for example, either (i) aA, aC, aG, and aT, or(ii) aA, aC, aG, and aU).

It is also not intended that the composition be limited by the nature ofthe label. However, in one embodiment, each of said four differentnucleotide analogues comprises a unique (preferably cleavable) label,said label selected from the group consisting of BODIPY, Rhodamine,Carboxyrhodamine, and Cyanine (see FIG. 36, which shows these labels inthe context of a cleavable disulfide bond).

It is also not intended that the composition be limited by the chemistryof the removable chemical moiety, which may, by way of example, comprisea disulfide bond or an azido group (e.g. an azidomethyl ether). Thechemistry may be the same or different vis-à-vis the cleavable linker.For example, said removable chemical moiety comprises an azido group andsaid cleavable linker comprises a disulfide bond.

In one embodiment, the present invention contemplates a compositioncomprising i) a first plurality of nucleotide analogues comprising fourdifferent (for example, in one embodiment, the four nucletodes areeither (i) aA, aC, aG, and aT, or (ii) aA, aC, aG, and aU) nucleotideanalogues, wherein each different nucleotide analogue is labeled with aunique (preferably cleavable) label and contains a removable chemicalmoiety capping the 3′-OH group; and ii) a second plurality of nucleotideanalogues comprising four different (for example, in one embodiment, thefour nucletodes are either (i) aA, aC, aG, and aT, or (ii) aA, aC, aG,and aU) nucleotide analogues, wherein each nucleotide analogue isunlabeled and contains a removable chemical moiety capping the 3′-OHgroup. Again, this composition may further comprise a polymerase and itis preferred that the reagents (e.g. said nucleotide analogues andoptionally said polymerase) are in solution.

It is not intended that the composition be limited by the particularlinkages. However, in a preferred embodiment, the nucleotide analoguesselected from the group consisting of cytosine, thymine, deaza-adenineand deaza-quanine and each comprising a unique (preferably) labelattached through a cleavable linker to a 5-position of cytosine orthymine or to a 7-position of deaza-adenine or deaza-guanine.

In one embodiment, the second plurality of nucleotide analogues ispresent in said solution at a high concentration than said firstplurality of nucleotide analogues. In one embodiment, said secondplurality of nucleotide analogues is present at a concentration between1 uM and 100 uM. In one embodiment, said first plurality of nucleotideanalogues is present at a concentration between 1 nM and 1 uM.

In one embodiment, the present invention contemplates kits, said kitscomprising a mixture of labeled and unlabeled nucleotide analogues(preferably both containing groups capping the 3′-OH—such as an azidogroup) and (optionally) polymerase. In one embodiment, the presentinvention contemplates a mixture of 4 labeled and 4 unlabeled nucleotideanalogues as herein described) and (optionally) polymerase. The mixturecan be provided dry or in solution in the kit (along with appropriateinstructions for extension reactions). Preferably, the unlabelednucleotide analogues are present in the mixture in a greater amount thanthe labeled nucleotide analogues.

The above-indicated solutions provide advantages in incorporationreactions. Thus, in one embodiment, the present invention contemplates amethod of incorporating labeled nucleotides into nucleic acid,comprising: a) providing i) a reaction chamber comprising plurality ofnucleic acid template molecules bound to a solid support, ii) a solutioncomprising a first plurality of nucleotide analogues wherein eachnucleotide analogue is labeled with a unique (preferably cleavable)label and contains a removable chemical moiety capping the 3′-OH group,and a second plurality of nucleotide analogues wherein each nucleotideanalogue is unlabeled and contains a removable chemical moiety cappingthe 3′-OH group; and iii) polymerase; b) introducing said solution intosaid reaction chamber under conditions wherein a nucleotide analogue ofsaid first plurality of nucleotide analogues is incorporated by saidpolymerase (e.g. the polymerase can be added separately or together withother reagents; regardless, it is preferred that said polymerase is insaid solution prior to step b); and c) detecting the label of theincorporated nucleotide analogue. The method may comprise additionalsteps (cleavage of the capping group, washing, etc.) and may repeatsteps (e.g. in order to incorporate subsequent, e.g. a second, third,fourth, etc., nucleotide analogues).

It is not intended that the present invention be limited by where thefirst (or subsequent) nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

In one embodiment, the second plurality of nucleotide analogues ispresent in said solution at a high concentration than said firstplurality of nucleotide analogues. In one embodiment, said secondplurality of nucleotide analogues is present at a concentration between1 uM and 100 uM. In one embodiment, said first plurality of nucleotideanalogues is present at a concentration between 1 nM and 1 uM.

In a preferred embodiment, said first plurality of nucleotide analoguescomprises four different nucleotide analogues and said second pluralityof nucleotide analogues comprises four different nucleotide analogues.In one embodiment, each of said four different nucleotide analogues ofsaid first plurality of labeled analogues comprises a unique label, saidlabel selected from the group consisting of BODIPY, Rhodamine,Carboxyrhodamine, and Cyanine.

Again, it is not intended that the present invention be limited by thenature of the chemical moiety capping the 3′-OH on the nucleotideanalogue or the (preferably cleavable) linker attaching the label. Inone embodiment, said removable chemical moiety comprises a disulfidebond. In one embodiment, said removable chemical moiety comprises anazido group (e.g. an azidomethyl ether). In one embodiment, saidremovable chemical moiety comprises an azido group and said cleavablelinker comprises a disulfide bond. In another embodiment, thesechemistries are reversed. Again, it is preferred that said moietycapping the 3′-OH is not a fluorescent moiety.

Increasing the number of bases that can be sequenced, i.e. increasingread lengths is desirable. However, as one proceeds to larger and largerread lengths, one often encounters a reduction in signal. In oneembodiment, the present invention contemplates reducing extension times(e.g. extension times of 5-15 minutes are reduced to 1-2 minutes, orless) in order to maintain signal strength at longer read lengths(greater than 20 bases, more preferably greater than 30 bases, etc.).This reduction in extension times can be combined with other methodsherein described (e.g. the use of mixtures of labeled and unlabelednucleotides) to improve performance and increase the retention insignal. Signal retention is defined as the ratio of signals at the endof the run to the signals at the beginning of the run.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows one embodiment of the imaging system of thepresent invention, said embodiment comprising a) a circular array ofLEDs configured such that the emitted light converges on a region orplatform (e.g. a position for a sample, flow cell, etc.) so as to excitefluorescence of fluorescent material, b) a lens assembly positionedabove the region so as to capture at least a portion of saidfluorescence, c) a filter wheel comprising bandpass filters, and d)light collection means (in this case a cooled CCD camera), wherein saidfilter wheel is positioned between the region where the light convergesand the light collection means.

FIG. 2 schematically shows one embodiment of a flow cell (200). FIG. 2Ashows a three dimensional translucent view of a flow cell, comprisingfluid tubing connections, cartridge heaters, and O-ring seal. FIG. 2B isa two dimensional drawing of a side view of a flow cell (200), showingan array or slide (201) with spaced spots on the surface (representingpositions for biomolecules and/or anchoring molecules), said arraypositioned (e.g. inverted) in a fluid channel (202) such that solutionsof buffers and/or reagents can be introduced over the surface underconditions whereby reactions and/or washing can be achieved. The arrowsshow one particular direction of fluid flow, with entrance (204) andexit ports (205), as well as one particular method of sealing (O-ringseal 203). FIG. 2C is a drawing depicting beads (206) in the wells (207)of the slide (or chip 208), which preferably comprises nucleic acid tobe sequenced (not shown), said slide positioned in a flow cell forcontact with reagents in the solution traveling through the flow cell.The single dark arrow shows reagent flow in the flow cell. The manylight arrow heads represent detection (e.g. light imaging) from the backof the slide (or chip).

FIG. 3A schematically shows one embodiment of a fluidics system (300),comprising a variety of illustrative reagent and buffer reservoirs incommunication (via tubing (306) or other channeling into a manifoldcomprising valves (305)) with one embodiment of a flow cell (comprisinga side entrance port (301) and one or more heaters 302), wherein thearray or chip (303) is inverted and the exit port (304) is on thebottom, thereby permitting the fluid channel to be drained at least inpart by gravity so that waste can be readily collected into a reservoir.3B shows another embodiment of the system (310), showing the flow cell(311) in relationship to the illumination and optics (312).

FIG. 4 shows a schematic for steps involved in sample preparation (A)and highly parallel sequencing steps (B) for embodiments of theinvention.

FIG. 5 shows a general structure of embodiments of cleavable fluorophorenucleotide conjugates with reversible terminator functionality. The3′-OH group is reversibly blocked by an allyl ether function and thefluorophore is attached via a cleavable allyl carbamate linker (bothshown in frames). After incorporation and signal readout, thefluorophore and the 3-O-allyl protective groups are cleaved by aqueoussolution of Pd (O).

FIG. 6 shows a cleavage mechanism for trimethyl lock based compounds.

FIG. 7 shows a cleavage mechanism for 1,6-rearrangement based compounds.

FIG. 8 is a schematic flow chart for re-phasing.

FIG. 9 shows simulated data showing sequence lead due to incorporationof small amounts of non-terminated nucleotides mixed with the reversiblyterminated nucleotides. Panel A shows actual fluorescent levels, Panel Bshows measured fluorescent levels.

FIG. 10 shows simulated data showing sequence lag due to finiteincorporation efficiency.

FIG. 11 is a chart of an exemplary sequence of extension events using anexemplary 4 templates positions and 3 cycles.

FIG. 12 is a chart of an exemplary sequence of extension events.

FIG. 13 shows a first portion of the chart of FIG. 11.

FIG. 14 shows a second portion of the chart of FIG. 11.

FIG. 15 shows data generated from the iterative application of equations1-3 using parameters in Table 1.

FIG. 16 panel A shows simulated base read data with 10% noise added andlead and lag factors of 1% each, and panel B shows reconstructed datawith the lead and lag removed.

FIG. 17 panel A shows simulated base read data with 10% noise added andlead and lag factors of 1.75% each, and panel B shows that attemptedreconstruction is poor as the lead/lag matrix is ill-conditioned, and C)shows reconstructed data with the lead and lag removed for only thefirst 18 bases. The 18-base lead/lag matrix is relatively well behavedand a more precise reconstruction may be performed.

FIG. 18 shows lead/lag matrix conditions for various lead and lagparameters for a 35 base read. In one embodiment, a condition numberbelow 20 produces accurate reconstruction.

FIG. 19 shows exemplary read length for various values of lead and lag.

FIG. 20 shows synthesis of 3′-O-azidomethyl-dNTPs where the steps denotetreatment with (i) DMSO, AcOH, Ac₂O, 48 h; (ii) SO₂Cl₂, dry CH₂Cl₂, 1-2h; (iii) NaN₃ in DMF, 3 h; (iv) NH₄F in MeOH, 16-20 h; (v) (MeO)₃PO,POCl₃ then (t-Bu₃NH)₄P₂O₇, TEAB, 1 h; vi) NH₄OH.

FIG. 21 shows synthesis of 3′-O-azidomethyl-dGTP where the steps denotetreatment with (i) DMSO, AcOH, Ac₂O, 48 h; (ii) Ph₂NCOCl, DIEA, Pyridine3 h; (iii) SO₂Cl₂, dry CH₂Cl₂, 1-2 h; (iii) NaN₃ in DMF, 3 h; (iv) NH₄Fin MeOH, 24 h; (v) (MeO)₃PO, POCl₃ then (t-Bu₃NH)₃P₂O₇H, TEAB, 1 h; (vi)NH₄OH.

FIG. 22 shows synthetic DNA templates used in exemplary sequencingexperiments.

FIG. 23 shows the structures of exemplary labeled2,3′-dideoxynucleotides used in the sequencing by synthesis.

FIG. 24 shows sequencing results using four different 25 nt DNAtemplates.

FIG. 25 shows synthesis steps for 2′-fluoro-3′-O-azidomethyl-dNTPs,where the steps comprise the following exemplary conditions (i) DMSO,AcOH, Ac₂O, 48 h; (ii) SO₂Cl₂, dry CH₂Cl₂, 1-2 h; (iii) NaN₃ in DMF, 3h; (iv) NH₄F in MeOH, 16-20 h; (v) (MeO)₃PO, POCl₃ then (t-Bu₃NH)₄P₂O₇,TEAB, 1 h; vi) NH₄OH.

FIG. 26 shows synthesis steps for2′-fluoro-3′-O-azidomethyl-(propargylamino)-dNTP synthesis.

FIG. 27 shows exemplary nucleotide structures with 3′-OH groupprotection that can be cleaved by mild oxidation reactions.

FIG. 28 shows an exemplary general synthetic pathway to install 3′-Oamino hemiacetal group (—CH₂ONH₂) and conversion to nucleotides.

FIG. 29 shows an exemplary synthetic pathway to prepare 3′-O carbazate(—CH₂ONH₂) nucleotide analogues

FIG. 30 shows an exemplary mechanism of 3′-O amino hemiacetal (—CH₂ONH₂)nucleotides deprotection reaction to generate free 3′-OH group.

FIG. 31 shows an exemplary mechanism of 3′-O-carbazate (—C(O)NHNH₂)nucleotides deprotection. The reaction may be fast due to higher entropycontribution of the leaving molecular nitrogen and carbon dioxide gas.

FIG. 32 shows sequencing by synthesis probe intensity in four channels(blue, green, yellow, and red) for a spot on a chip. Panel A is rawdata, and panel B is data with the color crosstalk removed.

FIG. 33 panel A shows 16-base-long sequence data, and panel B shows thesame data after applying the lead/lag compensation algorithm.

FIG. 34 panel A shows 25-base-long sequence data and panel B shows thesame data after applying the lead/lag compensation algorithm.

FIG. 35 provides examples of chemical structures of the reversiblyterminating nucleotides used in sequencing. These examples include:3′-O-azidomethyl nucleotides, 3′-O-aminoxy nucleotides, 3′-O-allylnucleotides; and disulfide nucleotides.

FIG. 36 provides examples of dyes conjugated to reversibly terminatingnucleotides via a cleavable linker.

FIG. 37 provides examples of compounds useful as cleaving agent“scavengers.”

FIG. 38 shows detection of incorporated nucleotides in an extensionreaction done in the presence of a first scavenger (cystamine).

FIG. 39 shows detection of incorporated nucleotides in an extensionreaction done in the presence of a second scavenger (ATA).

FIG. 40 is a schematic showing one embodiment for the synthesis of3′-O-azidomethyl,7-propargylamido-[3-((2-amidoethyl)dithio)propionamido]-6-carboxy-X-rhodaminedeoxyadenosine triphosphate.

FIG. 41 is a schematic showing one embodiment of a hot embossingtechnique for making slide (or chips) with indentations (which canreceive millions to billions of microbeads comprising nucleic acid).

FIG. 42 is a schematic comparing the structure of natural DNA with DNAthat was labeled with cleavable terminating nucleotides and then thelabel was removed. In this particular schematic, the example showspropargylamino derivatives.

FIG. 43 is a schematic showing a capping step to neutralize the reactivegroups after dye cleavage. For amines, one example that could be used isacetylation (such as acetic acid NHS ester); for the thiols(SH)N-methyl-maleimide or iodoacetamide can be used.

FIG. 44 shows the fluorescence signal from incorporated nucleotideanalogues observed as a function of the composition of the extensionmixture. In this case labeled nucleotides (3′-O-allyl) were supplementedwith up to 1 equivalent of non-labeled terminators (also 3′-O-allyl).The extension was performed and the resulting signal measured. Theresponse is different for different nucleotides tested and is a functionof polymerase bias.

FIG. 45 shows the results for two subsequent extensions performed on 4different DNA template. For extension 1, various amounts of labeledreversible terminating nucleotides were used (0, 50% and 100%). Aftercleavage, second extension was performed and the resulting signals weremeasured (bars on the right in each set). As can be seen the use of 100%labeled nucleotides in cycle 1 reduces the signal in subsequent cycle toby 50% compared to non-labeled reversible terminators.

FIG. 46 shows that improvements in sequencing performance can beachieved with a mixture of labeled/unlabeled nucleotides. Using such anucleotide mixture results in correct base calls.

FIG. 47 shows the signal decline observed using labeled nucleotides insequencing.

FIG. 48 shows that using a mixture of labeled and unlabeled nucleotides(e.g. a mixture of labeled and non-labeled reversible terminators) andcontrolling extension time can improve performance (e.g. increaseretention of signal) on an automated sequencing device. With additionalcontrol provided (e.g. by reducing extension time from 15 minutes to 2minutes), the incorporation rate of labeled nucleotides can becontrolled and results in improved fidelity and performance.

FIG. 49 shows that using a mixture of labeled and unlabeled nucleotides(e.g. a mixture of labeled and non-labeled reversible terminators) andcontrolling extension time can improve performance (e.g. increaseretention of signal) on an automated sequencing device. With additionalcontrol provided (e.g. by reducing extension time from 10 minutes to 1minute), the incorporation rate of labeled nucleotides can be controlledand results in improved fidelity and performance. Signal retention isdefined as the ratio of signals at the end of the run to the signals atthe beginning of the run.

FIG. 50A shows embodiments wherein hydroxymethyl modified base moietiesare employed to attach cleavable dyes (e.g. fluorescent markers and thelike) to the base. FIG. 50B shows the structures of hydroxymethylderivatives after cleavage of the dye and the 3′-O protective group.

DEFINITIONS

To facilitate understanding of the invention, a number of terms aredefined below, and others are found elsewhere in the specification.

The term “plurality” means two or more.

The term “nucleotide sequence” refers to a polymer comprisingdeoxyribonucleotides (in DNA) or ribonucleotides (in RNA).

The term “interrogation position” when made in reference to a nucleotidesequence refers to a location of interest in the sequence, such as thelocation at which the identity of a nucleic acid is sought to bedetermined.

The term “preceding nucleic acid” when made in reference to a firstnucleic acid in relation to a second nucleic acid that is located at aninterrogation position in a nucleotide sequence refers to a nucleic acidthat is inserted during synthesis into the nucleotide sequence beforethe insertion of the second nucleic acid at the interrogation position.The term “subsequent nucleic acid” when made in reference to a thirdnucleic acid in relation to the second nucleic acid at the interrogationposition refers to a nucleic acid that is inserted during synthesis intothe nucleotide sequence after the insertion of the second nucleic acidat the interrogation position.

The terms “probe” and “label” are interchangeably used to describe achemical moiety that, when attached to a composition of interest, actsas a marker for the presence of the composition of interest. Probes areexemplified by fluorescent moieties such as 5-carboxyfluorescein,6-carboxyrhodamine-6G,N,N,N′,N′-tetramethyl-6-carboxyrhodamine, and6-carboxy-X-rhodamine. Probes also include a fluorescence energytransfer tag that comprises an energy transfer donor and an energytransfer acceptor. The energy transfer donor is exemplified by5-carboxyfluorescein and cyanine, and the energy transfer acceptor isexemplified by dichlorocarboxyfluorescein,dichloro-6-carboxyrhodamine-6G,dichloro-N,N,N,N′-tetramethyl-6-carboxyrhodamine, anddichloro-6-carboxy-X-rhodamine. The mass tag is exemplified by a2-nitro-a-methyl-benzyl group, 2-nitro-a-methyl-3-fluorobenzyl group,2-nitro-a-methyl-3,4-difluorobenzyl group, and2-nitro-a-methyl-3,4-dimethoxybenzyl group.

The term “probe corresponds to a nucleotide” means that the probe servesas a marker for the presence of the nucleotide. Thus, detecting thepresence of the probe also detects the presence of the nucleotide.

The term “field flattening” when in reference to pixel intensity of animage refers to reducing differences in pixel intensity between two ormore pixels at different spatial locations on the image of a uniformlyradiating surface.

The terms “reducing,” “decreasing” and grammatical equivalents when inreference to the level of a molecule and/or phenomenon (e.g., lightintensity, chemical concentration, correlation between two event, etc.)in a first sample relative to a second sample, mean that the quantity ofmolecule and/or phenomenon in the first sample is lower than in thesecond sample by any amount that is statistically significant using anyart-accepted statistical method of analysis. In some embodiments, thequantity of molecule and/or phenomenon in the first sample is at least10% lower than, at least 25% lower than, at least 50% lower than, atleast 75% lower than, and/or at least 90% lower than the quantity of thesame molecule and/or phenomenon in a second sample. The term “reducing”includes, but does not require, a 100% lowering in the quantity of themolecule and/or phenomenon in the first sample compared to the secondsample.

The terms “increasing,” “elevating” and grammatical equivalents when inreference to the level of a molecule and/or phenomenon (e.g., lightintensity, chemical concentration, correlation between two event, etc.)in a first sample relative to a second sample, mean that the quantity ofmolecule and/or phenomenon in the first sample is higher than in thesecond sample by any amount that is statistically significant using anyart-accepted statistical method of analysis. In some embodiments, thequantity of the molecule and/or phenomenon in the first sample is atleast 10% greater than, at least 25% greater than, at least 50% greaterthan, at least 75% greater than, and/or at least 90% greater than thequantity of the same molecule and/or phenomenon in a second sample.

“Spectral” is a term that refers to electromagnetic radiation. In oneembodiment, the electromagnetic radiation is in the visible light region(wavelength of approximately 400-700 nanometers), such as that emittedby fluorescent moieties.

The terms “spectral filter” and “color filter” are interchangeably usedto refer to a filter for detection of a particular range ofelectromagnetic wavelengths, such as in the visible region, thereby Theterms “spectral crosstalk” and “color crosstalk” refer to any phenomenonby which a spectral signal, or a digital signal that corresponds to aspectral signal, that is transmitted and measured in one channel oftransmission creates an undesired effect in another channel. Forexample, spectral crosstalk may occur when exciting only a green dye,resulting in a signal that is visible in the yellow channel as well asin the green channel. Using methods disclosed herein, if this spectralcrosstalk is calibrated, it may be removed from subsequent measurementseven if the dyes are mixed in unknown quantities.

The term “low pass filter” refers to a filter that passes slowlyspatially varying intensity signals but reduces signals with higherspatial variation than a desired cutoff value. Exemplary software forcarrying out these steps is shown Appendix C, which is a source code forcreating a flat map calibration image.

The term “computer readable medium” refers to a medium, such as acompact optical disc, that is used to store and retrieve digital data.

One element is in “fluid communication” or “fluidic communication” withanother element when it is attached through a channel, tube or otherconduit that permits the passage of liquid, gas, vapor and the like.“Tubing” can be made of a variety of materials, including put notlimited to various plastics, metals and composites. Tubing can be rigidor flexible. Tubing can be “attached” in a detachable mode or a fixedmode. Tubing is typically attached by sliding into or over (both ofwhich are examples of “slidably engaging”) other tubing or connectors.

DESCRIPTION OF THE INVENTION

For further clarity, the invention is described below under thefollowing headings

A. Sequencing By Synthesis; B. Device Embodiments and Elements; C.Nucleotides; D. Reducing Lead And Lag; E. Dephasing; F. FieldFlattening; G. Spot Location in the Array; H. Image Sharpening; I. SpotBrightness Determination; J. Neighbor Influence Elimination; K. SpectralCrosstalk Calibration; L. Base Calls; and M. Software Appendices A-C

A. Sequencing by Synthesis

The invention relates to methods and compositions for determining theidentity of nucleic acids in nucleotide sequences using, for example,data obtained from sequencing by synthesis methods. Methods of DNAsequencing are generally described in Metzker, Genome Res. (2005)15(12): 1767-1776 and Shendure et al. (2004) Nature Reviews Genetics 5:335-344. The Sanger sequencing method or chain termination or dideoxymethod is a technique that uses an enzymatic procedure to synthesize DNAchains of varying length in different reactions that contain dilutedconcentrations of individual dideoxy nucleotides mixed in with normalnucleotides. DNA replication is stopped at positions that are occupiedby one of the dideoxy nucleotide bases resulting in a distribution ofnucleotide fragments since the normal nucleotides will properlyincorporate. Unnatural ddNTP terminators replace the OH with an H at the3′-position of the deoxyribose molecule and irreversibly terminate DNApolymerase activity. The resulting fragment lengths are determined todecipher the ultimate sequence. Electrophoretic separation of thedeoxyribonucleotide triphosphate (dNTP) fragments may be accomplishedwith single-base resolution.

In sequencing by synthesis, nucleotides conjugated with fluorescentmarkers that incorporate into a growing double-stranded nucleic acidfrom the single strand are detected. For example, one may immobilizetemplate DNA on a solid surface by its 5′ end. One may accomplish thisby annealing a sequencing primer to a consensus sequence and introducingDNA polymerase and fluorescent nucleotide conjugates (alternatively, aself-priming hairpin can be introduced by PCR or ligation to thetemplate). One detects nucleotide incorporation using a laser microarrayscanner or fluorescent microscope by correlating a particularfluorescent marker to a specific nucleotide. After each nucleotide isincorporated and the fluorescent signal is detected, one bleaches orremoves the fluorescent moiety from the nucleotide conjugate so as toprevent the accumulation of a background signal.

In one embodiment, the present invention contemplates DNA sequencing bysynthesis using an automated instrument, as well as methods andcompositions useful for sequencing using such an instrument. In oneembodiment, the instrument comprises a flow cell (FIGS. 2A and 2B) withat least two fluidics ports, a substrate with sequenceable nucleic acidmolecules attached to the substrate, reagent and waste reservoirs andfluidic system connecting the reservoirs to the flowcell (FIG. 3). Theflowcell is interfaced with a detection system to monitor theincorporation of the nucleotides.

In one embodiment, the sequencing by synthesis is carried out usingreversibly terminating nucleotides and polymerase. The nucleotidescomprise a protective group at their 3′-OH which prevents multipleincorporations and allows for accurate decoding of the sequence. Onceincorporated the protective groups can be cleaved with high efficiencyand specificity to allow subsequent nucleotide incorporations. Thenucleotides may also comprise a detectable label which can be cleavedafter the detection.

In one embodiment, the present invention contemplates a series of methodsteps, which an instrument for automated sequencing by synthesis maycarry out. In one embodiment, the process is comprised of the followingreagent reservoirs: 1) Extend A (reversibly terminated labelednucleotides and polymerase); 2) Extend B (reversibly terminatedunlabeled nucleotides and polymerase, but lacking labeled nucleotideanalogues); 3) Wash solution 1 (e.g. in one embodiment comprising adetergent, such as polysorbate 20, in a citrate solution, such as salinesodium citrate); 4) Cleave solution; 5) Wash solution 2 (e.g. in oneembodiment, comprising a detergent, such as polysorbate 20 in a buffercomprising tris(hydroxymethyl)aminomethane or “Tris”). Of course, thepresent invention is not limited to particular concentrations ofreagents in these solutions (and other buffers and detergents can beemployed). Nonetheless, in order to achieve high throughput rates, theincorporation reactions and the cleavage reactions are desired to befast. In one embodiment, high reaction rates are achieved by increasingthe concentration of reagents, agitation, pH or temperature (or thecombination of all these factors). The incorporation rate in addition isdependent on the specific activity and processivity of the polymeraseused. In one particular embodiment (which is provided by way of anon-limiting example), the reagents solutions have the followingcompositions and concentration ranges:

1) Extend A—reversibly terminated (3′-O-Azidomethyl) labeled (1 nM to 1uM) and non-labeled nucleotides (1 uM to 100 uM) and a first polymerase(1-500 ug/ml)); 2) Extend B—reversibly terminated non-labelednucleotides (1 uM to 100 uM) and a second polymerase (1-500 ug/ml)); 3)Wash solution 1 (3×SSC, 0.02% Tween 20); 4) Cleave solution (50-100 mMTCEP); 5) Wash solution 2 (100 mM Tris-HCl, 0.02% Tween 20, 10 mM KCl,20 mM (NH2)₂SO₄. In one embodiment, the first polymerase incorporateslabeled nucleotides better than the second polymerase, whichincorporates unlabeled nucleotides more efficiently. Examples ofcommercially available polymerases that can be used include TherminatorI-III. These polymerases are derived from Thermococcus sp. and carrymutations allowing for incorporation of modified nucleotides. Examplesof these polymerases are listed in Table below:

Therminator I NEB cat. # 9° N A485L (exo-) DNA Polymerase M0261LTherminator II NEB cat. # 9° N A485L/Y409V (exo-) DNA M0266L PolymeraseTherminator III NEB cat. # 9° N L408S/Y409A/P410V (exo-) DNA M0333LPolymeraseOther polymerases derived from 9 deg N parent polymerase or Thermococcussp. could also be used. Other suitable polymerase families couldconceivably be used after introducing mutation controlling the stericgate and enabling reversible terminators incorporation.

In one embodiment, the sequenceable DNA (preferably loaded on the chipor slide) is subjected to these solutions and compositions in theinstrument, and the sequencing is performed using automated protocol.Again, it is not intended that the present invention be limited to aprecise protocol or series of method steps. The order and number ofsteps can vary, as well as the time taken for each step. By way of anon-limiting example, in one embodiment, the instrument protocolcomprises (and is configured) as follows:

1. Extend A—0.5-5 minutes (delivery+agitation)

2. Extend B—1-20 minutes (delivery+agitation)

3. Wash 2—5-10 minutes (10-20× delivery and agitation followed by flowcell evacuation)

4. Image

5. Cleave—1-5 minutes (delivery+agitation)

6. Wash 1-5—10 minutes (10-20× delivery and agitation followed by flowcell evacuation)

7. Wash 2-5—10 minutes (10-20× delivery and agitation followed by flowcell evacuation)

8. Go to step 1

The cycle may be repeated as desired and images may be taken andsubsequently analyzed to decode the DNA sequence present at eachlocation.

In one embodiment of the above indicated cycle, eight nucleotideanalogues are employed: four (A, T, C, G) cleavably labeled andreversibly terminated; four (A, T, G, C) unlabeled but reversiblyterminated. In one embodiment, the concentration of the labeledanalogues is at a relatively low concentration [e.g. just enough to beincorporated into a substantial portion (e.g. so the label is visibleand detected) of the plurality of primers, whether they be detachedprimers or self-priming hairpins on the template]. By contrast, theunlabeled analogues, in one embodiment, are employed in a relativelyhigh concentration (e.g. in order to drive the extensions to completion,and avoid primers, whether they be detached primers or self-priminghairpins, that lack incorporation of a first nucleotide analogue). Ithas been found empirically that the use of unlabeled nucleotidesimproves read lengths, and reduces lead and lag (discussed below).

One example of a currently optimized protocol running on Beta instrumentusing 3′-O-azidomethyl/disulfide labeled nucleotides and non-labeled3′-O-azidomethyl nucleotides is shown

Nucleotide Labeled nucleotides [nM] Un-labeled nucleotides [nM] dCTP 30250 dATP 20 250 dGTP 30 250 TTP 30 250in the Table (above), wherein un-labeled nucleotides are employed inratios between 8.33 to 1 and 12.5 to 1 (relative to labelednucleotides). In one embodiment, the labeling (i.e. incorporation) stepuses Kapa RevTerm polymerase (from Kapa Biosystems, Woburn, Mass.) at 2μg/ml and is performed at 55 deg C. for 1-2 minutes. This is followed bysynchronization step where only non-labeled nucleotides are used at 25μM concentration and a polymerase derived from 9 deg N (Thermococcussp). at 25 μg/ml is used. This step is also carried out at 55 deg C.Thus, unlabeled nucleotide analogues can be employed together withlabeled nucleotides, as well as in steps where no labeled nucleotidesare employed.

B. Device

In one embodiment, the present invention contemplates using an opticalsystem, for exciting and measuring fluorescence on or in samplescomprising fluorescent materials (e.g., fluorescent labels, dyes orpigments). In a further embodiment, a device is used to detectfluorescent labels on nucleic acid. In another embodiment, the devicecomprises a fluorescent detection system and a flow cell for processingbiomolecules (e.g., nucleic acid samples) arrayed on a “chip” or othersurface (e.g., microscope slide, etc.). The flow cell permits the userto perform biological reactions, including but not limited to,hybridization and sequencing of nucleic acids.

It is not intended that the present invention be limited to particularlight sources. By way of example only, the system can employultra-bright LEDs (such as those available from Philips LumiledsLighting Co., San Jose, Calif.) of different colors to excite dyesattached to the arrayed nucleic acids. These LEDs are more costeffective and have a longer life than conventionally used gas or solidstate lasers. Other non-lasing sources of lights such as incandescent orfluorescent lamps may also be used.

FIG. 1 shows a useful configuration of the LEDs, whereby the emittedlight converges on a region or platform (e.g., suitable for positioningthe flow cell or sample). However, linear arrays of LEDs can also beused.

It is not intended that the present invention be limited to particularlight collection devices. By way of example only, the system may employa high sensitivity CCD camera (such as those available from RoperScientific, Inc., Photometric division, Tucson Ariz. or those availablefrom Apogee Instruments, Roseville, Calif.) to image the fluorescentdyes and make measurements of their intensity. The CCD cameras may alsobe cooled to increase their sensitivity to low noise level signals.These may also be CMOS, vidicon or other types of electronic camerasystems.

Since LED illumination light is not a collimated beam as from lasers, itis therefore an appropriate choice for imaging a larger area of manynucleic acid spots. To get sufficient light and therefore fluorescentsignals over the larger area, the area seen by each pixel of the cameramust be of sufficient size to allow enough fluorescent dye molecules tocreate a sufficient signal (for example, an Apogee U13 CCD available has1.3 megapixels of 16 microns in size, while the Apogee U32 has 3.2megapixels of 6.8 microns in size).

To increase capacity and efficiency, the present invention contemplatesin one embodiment, a two flow cell system (e.g. while one chip in afirst flow cell is undergoing one or more reaction steps, a second chipin a second flow cell is being scanned and imaged) with a single camera.In yet another embodiment of an imaging system, two flow cells and twocameras are employed.

In one embodiment, the chip containing the array of nucleic acid spotsis processed in a transparent flow cell incorporated within theinstrument, which flows reagent past the spots and produces the signalsrequired for sequencing (see FIGS. 2A and 2B). In a particularembodiment, the chip remains in the flow cell while it is imaged by theLED detector. The flow cell and associated reagents adds the nucleicacids, enzymes, buffers, etc. that are required to produce thefluorescent signals for each sequencing step, then the flow celldelivers the required reagents to remove the fluorescent signals inpreparation for the next cycle. Measurement by the detector occursbetween these two steps. In order for reactions to take place, the flowchannels are configured to be of sufficient dimensions. For example, theflow-cell fluid channel formed by the array and the flat surface of theflow cell are at least 0.1 mm in depth (more particularly 0.5 mm indepth) and the volume formed by the chip, the block and the seal is atleast 100 microliters in volume (more particularly, between 100 and 700microliters, and still more particularly, between 150 and 300microliters, e.g. 200 microliters, in volume).

In one embodiment, the flow cell is motionless (i.e., not moved duringreactions or imaging). On the other hand, the flow cell can readily bemounted on a rotary or one or more linear stages, permitting movement.For example, in a two flow cell embodiment, the two flow cells may moveup and down (or side to side) across the imaging system. Movement may bedesired where additional processes are desired (e.g., where exposure toUV light is desired for photochemical reactions within the flow cell,such as removal of photocleavable fluorescent labels), when multipleflow cells share a single camera, or when the field of view of thedetection system is smaller than the desired area to be measured on theflow cell. The detector system may also be moved instead of or inaddition to the flow cell.

In a further embodiment, the flow cell is in fluid communication with afluidics system (see illustrative system shown in FIG. 3. In oneembodiment, each bottle is pressurized with a small positive gaspressure. Opening the appropriate valve allows reagent to flow from thesource bottle through the flow cell to the appropriate collectionvessel(s). In one embodiment, the nucleotides and polymerase solutionsare recovered in a separate collection bottle for re-use in a subsequentcycle. In one embodiment, hazardous waste is recovered in a separatecollection bottle. The bottle and valve configuration allow the washfluid to flush the entire valve train for the system as well as the flowcell. In one embodiment, the process steps comprise: 1) flushing thesystem with wash reagent, 2) introducing nucleotides (e.g. flowing anucleotide cocktail) and polymerase, 3) flushing the system with washreagent, 4) introducing de-blocking reagent (enzyme or compounds capableof removing protective groups in order to permit nucleic acid extensionby a polymerase), 5) imaging, 6) introducing label removing reagent(enzyme or compounds capable of removing fluorescent labels), and 7)flushing the system with wash reagent.

The system can be made to include a user interface system. The Labview(National Instruments, Austin, Tex.) system is available and providessoftware for computer controlled systems. Galil Motion Control (Rocklin,Calif.) provides motion control systems that can be interfaced tocontrol the instrument.

C. Nucleotides

The invention's compositions and methods contemplate using nucleotidesequences that contain nucleotides. The terms “nucleotide” and “nucleicacid” refer to constituents of nucleic acids (DNA and RNA) that containa purine or pyrimide base, such as adenine (A), guanine (G), cytosine(C), uracil (U), or thymine (T)), covalently linked to a sugar, such asD-ribose (in RNA) or D-2-deoxyribose (in DNA), with the addition of fromone to three phosphate groups that are linked in series to each otherand linked to the sugar. The term “nucleotide” includes nativenucleotides and modified nucleotides.

“Native nucleotide” refers to a nucleotide occurring in nature, such asin the DNA and RNA of cells. In contrast, “modified nucleotide” refersto a nucleotide that has been modified by man, such as using chemicaland/or molecular biological techniques compared to the nativenucleotide. The terms also include nucleotide analogs attached to one ormore probes to facilitate the determination of the incorporation of thecorresponding nucleotide into the nucleotide sequence. In oneembodiment, nucleotide analogues are synthesized by linking a uniquelabel through a cleavable linker to the nucleotide base or an analogueof the nucleotide base, such as to the 5-position of the pyrimidines (T,C and U) and to the 7-position of the purines (G and A), to use a smallcleavable chemical moiety to cap the 3′-OH group of the deoxyribose orribose to make it nonreactive, and to incorporate the nucleotideanalogues into the growing nucleotide sequence strand as terminators,such as reversible terminators and irreversible terminators. Detectionof the unique label will yield the sequence identity of the nucleotide.Upon removing the label and the 3′-OH capping group, the polymerasereaction will proceed to incorporate the next nucleotide analogue anddetect the next base. Exemplary fluorescent moieties and fluorescentsemiconductor crystals are described in Ju et al., U.S. Pat. No.6,664,079, hereby incorporated by reference.

Other nucleotide analogs that contain markers, particularly cleavablemarkers, are also contemplated, such as those configured using allylgroups, azido groups, and the like, and which are further describedbelow. One of the preferred moieties used to attach cleavable linkers ispropargyl based or allyl based. Other means of attaching cleavablelinkers and dyes are also contemplated. In particular, attachments tothe base moiety that result with little or no residual linker after dyecleavage are particularly preferred. Attachments to the base that resultwith residual linkers after cleavage that do not carry charge are alsopreferred. These features are important to ensure that the nucleotidesare incorporated in the efficient manner by the enzyme into growingstrand of nucleic acid after the cleavage of the label/dye. Oneparticular embodiment contemplated by the present invention comprisesthe use of hydroxymethyl modified base moieties to attach cleavabledyes. Examples of such compounds are shown in FIG. 50A. FIG. 50B showsthe structures of hydroxymethyl derivatives after cleavage of the dyeand the 3′-O protective group.

The nucleotide compositions of the invention are particularly useful inmassively parallel DNA Sequencing By Synthesis (SBS) approachesutilizing fluorophores as markers.

a. Allyl Analogs

Cleavable fluorescent nucleotides with photo-cleavable linkers havingreversible terminator allyl groups have been described in Ruparel et al.(2005) Proc. Natl. Acad. Sci. 102(17) 5932-7. Similar, fluorescentnucleotide conjugates have been described in Bi et al. (2006) J. Am.Chem. Soc. 128(8) 2542-3. In one embodiment, the invention contemplatesusing nucleotide analogs with cleavable markers conveniently configuredwith allyl groups. In a particular embodiment, the exposed amine groupsof incorporated nucleotides are capped during sequencing. In otherembodiments, the nucleotide derivatives comprise two or more allylethers and synthetic intermediates thereto.

Sample preparation and parallel sequencing steps are exemplified, butnot limited, to those illustrated in FIGS. 4 and 5. FIG. 4 Panel A showshow one isolates and prepares the DNA prior to sequencing and Panel Bshows the sequencing cycle. One isolates DNA from a biological sourceand shears it by a mechanical device to the desired average size. Oneend-repairs, A-tails, and circularizes the fragments using a dT-tailedlinker about 100 nucleotides in length. The linker consists of twooutward directed primer recognition sequences and an arbitrary sequenceof about 100 bases between the priming sites. After ligation, onedecomposes noncircular sequences by treatment with an endonuclease. Onedilutes the circular DNA fragments to prepare them for bead-basedemulsion PCR using a biotinylated forward primer and a bead-attachedreverse primer carrying an azido group on its 5′-end. One performsemulsion PCR. An aqueous mix containing all the necessary components forPCR plus primer-bound beads and template DNA are stirred together withan oil/detergent mix to create microemulsions. The aqueous compartmentscontain an average of less than one template molecule and less than onebead. The microemulsions are temperature-cycled as in a conventionalPCR. If a DNA template and bead are present together in a single aqueouscompartment, the bead-bound oligonucleotides act as primers foramplification. One breaks the emulsion and subjects the mixture to anenrichment step by using streptavidin coated magnetic beads. Onedenatures the nucleic acid immobilized on the beads generating singlestranded amplicons to which a self-priming hairpin moiety is thenligated.

The beads are then arrayed on a chip surface and the sequencing bysynthesis reactions are performed. Each cycle comprises steps that areused to read out the DNA sequence (See FIG. 4, Panel B). One subjectsthe array segment to the fluorescent nucleotide conjugate with ahydroxyl-protecting group on the 3′ end. One scans the array and thefluorescent output of each of the fluorescent markers and measures theoutput for each position. One exposes the array to conditions forcleavage of the fluorescent marker and the hydroxyl-protecting group.The entire process is repeated with another set of nucleotide bases unitthe sequence of each position is determined. As the sequence data isgenerated, one collects the sequence information and aligns thereference sequences for diagnosis. One may use computer software and adatabase of previously known mutations and corresponding sequences tocorrelate them to the sequence with known mutations.

The PCR approach described above ensures that instead of sequencing ofthe entire pool of templates, one performs clonal or digital sequencing,resulting in much higher sensitivity for detection of mutations. Forexample, if a spontaneous mutation is present at only 5% of thepopulation and the remaining 95% of the gene copies are wild types it isdifficult to detect the mutated DNA using a conventional pool sequencingapproach because of insufficient sensitivity. In the applicants'approach, one dilutes the input sample so that each PCR emulsion bubblecontains at most a single template, which is then subjected tosequencing. If one performs this process on 1,000 unique clones, thenone on average detects mutant sequences (present in 5% of amplicons) in50 reactions and wild type sequence in 95% of the reactions.

b. Azido Analogs

Nucleotide analogs that contain cleavable markers configured using azidogroups are also useful in the invention's methods and compositions. Thenucleotide analogs are exemplified by nucleotide compositions comprisingcompounds of the following general structure:

Where PG1 stands for protective group that is selectively removable and,and CL stands for cleavable linker, which is also selectively cleavable,and R is selected from the group of H, OH, F, NH₂. Several particularembodiments of this invention are contemplated. In one embodiment thesenucleotide compositions can be incorporated into the nucleic acid bynucleic acids modifying enzymes in a controlled fashion to decode theidentity of the bases encoded by the marker moiety M. Once the identityof the base has been decoded, then the marker moiety can be cleaved offand removed. This invention contemplates the use of the cleavablelinkers based on the “trimethyl lock” mechanism or the“1,6-rearrangement” mechanism. The 3′-O-protective groups which act asreversible terminators can also be cleaved off to enable addition of thenext nucleotide. This invention contemplates the use of azidomethyl,methylaminoxy, disulfide and allyl groups as reversible 3′-OHterminators.

Methods for synthesizing exemplary nucleotide analogs that containcleavable markers configured using azido groups are described inExamples 2-11 and shown in FIGS. 20-26.

The invention contemplates the use of the cleavable linkers based on the“trimethyl lock” mechanism or the “1,6-rearrangement” mechanism. The3′-O-protective groups which act as reversible terminators can also becleaved off to enable addition of the next nucleotide. The inventioncontemplates the use of azidomethyl, aminooxy, methylaminoxy and allylgroups as reversible 3′-OH terminators.

i. Cleavable Linkers (Cl)

Cleavable linkers are exemplified by trimethyl lock based linkers and1,6-rearrengement linkers as further described below.

1. Trimethyl Lock Based Linkers

Cleavable linkers are the linkers linking the marker molecule M to thebase and these can be selectively cleaved using specific cleavingagents. Specifically, this invention contemplates the use of a“trimethyl lock” structure as the cleavage mechanism. These structuresare well known in the chemical arts and have been used before incontrolled drug release applications. The general structures ofcleavable trimethyl lock based linker utilized in particular embodimentsof the present invention are shown below:

The above shows exemplary embodiment A where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is a divalentgroup selected from NH, O, S.

The above shows exemplary embodiment B where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is NH.

The above shows exemplary embodiment C where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is a divalentgroup selected from NH, O, S, and Y is a selectively removableprotective group.

The above shows exemplary embodiment D where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, X is NH, and Y is anazidomethyl group.

The cleavage mechanism for the trimethyl lock based compounds is shownschematically in FIG. 6. This phenomenon has been previously describedin the chemical literature and used as for basic research studies(Borchardt and Cohen (1972). J. Am. Chem. Soc. 94(26): 9166-9174, Wanget al. (1996) Bioorg. Chem. 24: 39-49), as caging agents for controlleddrug delivery (Wang et al. (1997). J. Org. Chem. 62(5): 1363-1367) andas protective groups in organic synthesis (Wang et al. (1995). J. Org.Chem. 60(3): 539-543).

The linkers in the present invention leverage the ability of thetrimethyl lock system to create cleavably linked nucleotides.

2. 1,6-Rearrengement Linkers

The invention contemplates another category of cleavable linkers linkingthe detectable marker moiety to the nucleotide that are based on 1,6quinone methide rearrangement mechanism (Carl et al. (1981). J. Med.Chem. 24(5):479-480; Duimstra et al. (2005). J. Am. Chem. Soc. 127(37):12847-12855). These structures are well known in the chemical arts andthey have been used before for the controlled drug release applicationsand for chemical synthesis (Azoulay et al. (2006) Bioorganic & MedicinalChemistry Letters 16(12): 3147-3149; Murata et al. (2006) TetrahedronLetters 47(13): 2147-2150). The general structures of cleavable 1,6rearrangement mechanism based linker utilized in some embodiments of thepresent invention are shown below:

The above shows exemplary embodiment E, where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker and Y is a selectivelyremovable protective group.

The above shows exemplary embodiment F, where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker.

The above shows exemplary embodiment G where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is a divalentgroup selected from the following: NH, O, S.

FIG. 7 shows an exemplary cleavage mechanism for the cleavable linkerdescribed in the following exemplary embodiment G.

The above shows exemplary embodiment H where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is a divalentgroup selected from the following: NH, O, S. The cleavage is driven hereby the reducing agent and nucleophilic attack of the resulting aminogroup on the carbonyl followed by cyclization. This mechanism has beenused before for the development of protective groups for applications inthe carbohydrate and nucleoside chemistry (Wada et al. (2001).Tetrahedron Letters 42(6): 1069-1072; Xu et al. (2002) CarbohydrateResearch 337(2): 87-91).

The cleavable linker attachment to the base moiety can be achieved invariety of ways that are well known in the art. Among these is the useof linkers based on 1) propargylamino nucleosides, 2) aminoallylnucleosides, and 3) propargylhydroxy nucleosides.

ii. Protective Groups (PG1)

The invention contemplates nucleotide compositions comprising thefollowing protective groups (PG1) that reside on the 3′-OH groups of thenucleotides: 1) 3′-O-Azidomethyl ethers, 2) 3′-O-disulfide, 3)3′-O-methylaminoxy, and 4) 3′-O-allyl.

With respect to the 3′-O-Azidomethyl ethers, exemplary protective groupsthat reside on the 3′-OH groups of the nucleotides that are within thescope of this invention are 3′-O-azidomethyl groups. These groups can beremoved using mild reducing agents, such asTris(2-carboxyethyl)phosphine (TCEP).

With respect to the 3′-O-disulfide group, the 3′-O-disulfide group canbe removed under mild oxidative conditions, for example using in usingmild reducing agents, such as Tris(2-carboxy-ethyl)phosphine (TCEP).

With respect to the 3′-O-methylaminoxy group, the 3′-O-methylaminoxy(3′-O—CH2-NH2) group can be removed under mild oxidative conditions, forexample using in situ generated nitrous acid (such as from sodiumnitrite).

As to the 3′-O-allyl group, this protective group can be removed using avariety of reducing agents, including transition metal complexes (Pd,Rh).

c. 3′-O-Protected Nucleosides and Nucleotides

The invention contemplates compositions comprising compounds of thefollowing general structure:

PG1 stands for protective group that is selectively removable and, andCL stands for cleavable linker, which is also selectively cleavable. Inone embodiment these nucleotide compositions can be incorporated intothe nucleic acid by nucleic acids modifying enzymes in a controlledfashion for example to decode the identity of the bases encoded by themarker moiety M. Once the identity of the base has been decoded, thenthe marker moiety can be cleaved off and removed. In one embodiment, theinvention contemplates the use of cleavable protection for 3′-OH innucleotides for reversible terminators for SBS.

Examples of PG1 protective groups are shown in FIG. 27. As anillustration, the synthesis of one of the embodiments in such classes ofnucleotide-3′-O—(CH₂ONH₂)-dNTPs is presented in FIG. 28. Briefly, theprotected 3′-methylthiomethyl nucleoside (1) upon treatment with SO₂Cl₂produce activated product (2) which after reaction with hydroxylamine orits N-Fmoc protected compound install aminoxy group. The later compoundscan be triphosphorylated to result in nucleotides. Other compounds andexemplary synthesis pathways within the scope of the invention are shownin FIGS. 29-31.

D. Reducing Lead And Lag

The cleaving agent is designed to cleave the 3′-OH or the dye attachedto the nucleotide or both the 3′-OH and the dye. A variety ofchemistries may be used for these attachments. FIG. 35 shows variouspossible chemistries for the 3′-OH group. FIG. 36 shows disulfidelinkers for attaching the dye. Importantly, for any particularnucleotide, the chemistries may be same or may be different. Forexample, in one embodiment, the 3′-OH group can carry an azidomethylether and yet the dye can be attached via an azido linker. In anotherembodiment, however, the 3′-OH group can carry an azidomethyl ether andyet the dye can be attached via a disulfide linker. Both the azidomethylether and the disulfide linker are cleavable by TCEP(Tris-carboxyethyl)phosphine, although the disulfide linker cleaves muchfaster than the 3′-O-azidomethyl ether.

The cleaving agent is used at relatively high concentration (50-100 mM)to achieve fast cleavage. It is important for the sequencing process toremove any traces of cleaving agent in the wash steps, as these tracescould interact with the Extend A and B solution (see the discussion ofthese solutions above) in the next cycle and create native nucleotides.This is highly undesirable as this leads to sequence dephasing (lead andlag) and limits useful read lengths.

One approach might be to increase the number of washes. However, it hasbeen found empirically that increased washing cycles after cleavage stephave only minimal effect on the sequencing performance unless very highnumbers of washes are used (see Example 14). Such an approach would slowdown the process considerably.

The present invention, in one embodiment, contemplates a differentapproach to solving the problem. In one embodiment, the presentinvention contemplates novel compositions to be used in one or more ofthe solutions employed in the sequencing by synthesis method (or in anew, additional separate solution) that reduce, minimize and/or inhibitthe cleaving agent and the “pre-cleaving”effect. In one embodiment, acleavage agent “scavenger” is contemplated. The cleavage agent scavengeris designed to react with any leftover cleaving reagent remaining in theflow cell or the fluidics (e.g. tubing) by inefficient or incompletewashing. In one preferred embodiment, the scavenger agent is added tothe wash solution directly after the cleave step. In another embodimentthe scavenger is added to the Extend A solution. In yet anotherembodiment the scavenger agent is added to Extend B solution. Thescavenger requirements are as follows: 1) solubility; 2) fast andspecific reaction with the cleaving agent. In the embodiments where thescavenger is added to Extend A or B solution, there is the additionalrequirement of lack of inhibition of polymerase reaction and lack ofreactivity with functional groups on the nucleotides, dyes orpolymerase.

In one particular embodiment, the scavenger agent mimics the structureof the protective group present on the 3′-OH location of the nucleotide.In another embodiment, the scavenger mimics only the reactivity of theprotective group. For example, in case of 3′-O-azidomethyl nucleotidesscavenger compounds could comprise azidomethyl, azidoethyl ethers ordisulfide compounds. In case of 3′-O—NH2 nucleotides the scavengerscould be any aminoxy compounds, such as hydroxylamine. In case of3′-O-allyl nucleotides the scavengers could be any allyl ether ordisulfide compounds. FIG. 37 provides examples of cleaving agent“scavengers.” It has been found empirically (see Example 15), that theuse of such compounds improves base calling accuracy, without the needfor additional wash steps (and in particular, without the need for highnumbers of wash cycles).

E. Dephasing

Many next-generation DNA sequencing systems read the sequence ofmillions of different single-stranded DNA fragments in parallel by usinga polymerase enzyme to incorporate fluorescently labeled DNA nucleotidesinto the complementary strand one cycle at a time. However,incorporation errors can shift the phase of some of the templates, sobase read outs may lead ahead or lag behind the cycle number. Theinvention provides a model and methods to account for incorporationerrors and show how the model may be inverted to correct this dephasingand extend read lengths.

Although fluorescence-based, single-molecule sequencing on a chip hasbeen demonstrated, it is very sensitive to polymerase incorporationerrors. This may be reduced and therefore reliability of sequence readout may be increased if each spot on a chip is an ensemble of identicaltemplate molecules. Polymerase errors (such as the incorporation of thewrong complementary nucleotide or no incorporation at all) areinevitable, but infrequent. Therefore, the superposition of all of thefluorescent signals from template molecules within an ensemble willprimarily be from the correct nucleotide. As the number of cycles getslarge, however, certain errors can accumulate within an ensemble andcontribute to possible miscalling of the correct nucleotide.

For our analysis, we assume that a set of reversibly terminated andcleavably labeled nucleotides with four different dye colors (one foreach nucleotide type: A, C, G and T) are used for sequence read out. Themethods described herein may also be applied to other types of SBSprocesses such as pyrosequencing. If the SBS process works withoutmis-incorporations, then for each cycle only a single nucleotide type isincorporated into every strand in an ensemble. During a read out phase,the color of each ensemble is measured, then during a cleavage phase,the terminator and dyes are cleaved off and the chip is ready for thenext cycle. Thus, the position of the base being read out on everytemplate on the chip is synchronized with the cycle number.

Because of impurities, limited polymerase efficiencies and other errors,some of the templates within an ensemble may get out of phase with thecycle number. For example, the base that is incorporated in the i^(th)cycle may be complementary to the i-1^(st) position or the i+1^(st)position in the template rather than the expected i^(th) position. Theinvention's methods provide computational re-phasing of the dephaseddata. FIG. 8 is a schematic flow chart for re-phasing. Additional datademonstrating the efficacy of the invention's methods is discussedbelow, including FIGS. 9-19 and Example 11 (FIGS. 33-34).

d. Sequence Lead

Polymerases that have an increased capacity for incorporating 3′reversibly terminated nucleotide analogs continue to have a preferencefor incorporating native nucleotides. This means that even thoughnucleotide analogs may be extremely pure, any residual nucleotides with3′-OH (non-terminated) will be incorporated at a much higher rate andtherefore appear to be more prevalent. The incorporation ofnon-terminated nucleotides has the effect of skipping a base, as asecond incorporation (the next base) will occur in the same cycle. Thus,the fluorescent measurement for that template will exhibit the dye fromthe following base rather than the expected base at that cycle number.Since that template now exhibits a “lead,” it will continue to do so,even if all future nucleotides are reversibly terminated. This effect iscumulative and shown in simulated data in FIG. 9 for a non-terminatednucleotide incorporation rate of 2% as compared to the terminatednucleotide analog rate and a repeated 35 base sequence of ACTGACTGACTG .. . . Here we make the assumption that each of the nucleotides has thesame nonterminated incorporation rate thereby allowing us to use alinear model. Again, the actual nucleotide purity may well be betterthan say 99.5%, but the apparent non-terminated incorporation rate maybe 2% depending on the polymerase. In the example in FIG. 9, the modeltells us the amount of signal due to the sequence lead effect. In cycle20, the model calculates that we have 60% of the signal from the primarybase at the 20th position (red), 32.4% of the signal from the base atthe 21st position (blue), 6.7% of the signal from the base at the 22ndposition (green), 0.8% of the signal from the base at the 23rd position(yellow), and 0.07% of the signal from the base at the 24th position(red). An interesting observation is that with the lead effect, theprimary base signal (actual base at that cycle) does not have a 100%signal as some templates are already “reading out” subsequent bases onthat strand. Thus at the end of a run, we can “look forward” and shiftback the lead signals and correct the primary signals. We denote thecontributions at the i^(th) cycle as R_(0Lead,i), R_(+1Lead,i),R_(+2Lead,i), R_(+3Lead,i), etc. for the ratio between the reducedsignal for the i^(th) base to the actual i^(th) base population, theratio contribution to the i^(th) base signal from the i+1^(st) base, theratio contribution to the i^(th) base signal from the i+2^(nd) base,etc. Because the amount of lead changes with each cycle, there will be adifferent set of ratios for each cycle.

e. Sequence Lag

We developed a model for de-phasing due to sequence lag. This is causedby limited incorporation efficiency where some small percentage of thetemplates do not get a base incorporated in the cycle. FIG. 10 showssimulated data for a 98% incorporation efficiency for the same templatesequence as in FIG. 9. We denote the contributions at the i^(th) cycleas R_(0Lag,i), R_(1Lag,i), R_(−2Lag,i), R_(−3Lag,i), etc., for the ratiobetween the reduced signal for the i^(th) base to the actual i^(th) basepopulation, the ratio contribution to the i^(th) base signal from thei−1^(st) base, the ratio contribution to the i^(th) base signal from thei−2^(nd) base, etc.

f. Nucleotide Incorporation Events

As discussed above, every time there is an available site for thepolymerase to incorporate a nucleotide on a template, there are threepossible outcomes: First, no nucleotide is incorporated—Event No-I. Ifno nucleotide is incorporated due for example to polymeraseinefficiency, then the site remains available for the next cycle. Weterm this a “lag” event as it has the effect of causing a readout in thenext cycle that will from the position behind or lagging the cyclenumber. Second, a reversibly terminated nucleotide is incorporated—EventT-I. If as expected, a reversibly terminated nucleotide is incorporated,then the nucleotide readout is in sync with the cycle number. In thenext cycle, the next consecutive template nucleotide position will beavailable for incorporation. Third, a non-terminated (native) nucleotideis incorporated—Event N-I. If a non-terminated nucleotide isincorporated, then during that same cycle, there is a second opportunityfor another nucleotide to be incorporated at the subsequent position inthe template strand. We term this a “lead” event as it has the effect ofcausing a readout of a nucleotide that is at a position that is ahead ofor leading the cycle number. This second incorporation event is subjectto the same three possible outcomes (No-I, T-I or N-I); thus, N-I eventsare recursive.

We will use the variable G_(i) to represent the rate at which a lagoccurs at template position i and similarly D_(i) for the lead rate atposition i. The analysis assumes that these rates may vary from positionto position depending on the identity of the nucleotide that is to beincorporated, but we have assumed that all incorporation events for aparticular nucleotide have the same lag and lead rates, even if theincorporation is not the first one in a cycle (it follows an N-I event).The fluorescent signal that will be generated from an incorporationevent at a template location i is proportional to (1-G_(i)-D_(i)), so atevery i^(th) incorporation event, the three types of events (No-I, T-Iand N-I) will occur at the following rates: Event No-I at rate G_(i),Event T-I at rate (1-G_(i)-D_(i)), and Event N-I at rate D_(i).

g. Signals Produced in Each Cycle

Although there are only three potential outcomes from an incorporationevent, all of the combined events from multiple cycles in a template canbe fairly complex. FIG. 11 may be used to better visualize the sequenceof extension events. For simplicity, only 4 templates positions and only3 cycles are illustrated in FIG. 11. The numbered regions in thevertical direction along the left edge indicate the nucleotide positionalong the strand. The horizontal direction symbolizes the relativenumber of strands in an ensemble that undergo events No-I, T-I or N-I(lag, readout or lead). The various events for cycle 1 in the sequenceprocess are shown in shades of blue, events for cycle 2 are shown in redand events in cycle 3 are shown in green.

For clarity in FIG. 11, we have designated each of the three possibleevents (No-I, T-I and N-I) to occur at the same rate for every cycle. Inan actual system, the lead and lag rate are both likely to be muchsmaller values. The chart is easier to understand if it is viewed onecolor at a time. The blue regions represent events that occur during thefirst cycle. At position 1 of the template, the entire ensemble oftemplates are available for extensions, thus (light blue) undergo a lag(no incorporation), (medium blue) are read out and (dark blue) undergo alead. The portion of templates that experienced a lag (light blue)during the first cycle, remain available during the second cycle forincorporations. The portion of the templates that experienced a readout(medium blue) comprises the signal that is read during cycle 1 atposition 1. This portion will progress in synchrony and allowincorporations to occur at position 2 during the second cycle. Theportion of the templates that experienced a lead at position 1 will havea second incorporation event during cycle 1 at the second position ofthe template. This incorporation again will be split equally into thethree possible events. A portion of the templates will remain unextended(lag), a portion will generate a signal (readout) and a portion willundergo a lead and produce a third set of incorporation events atposition 3. This process will continue during cycle 1. Although leadevents may continue down the entire length of the template during cycle1, in practical terms, the effects after about 4 lead events arenegligible.

In cycle 2 (red colors), the only strands that are available to beextended at position 2 are those for which one of three events occurred(see FIG. 12): (1) strands that were read out at position 1 during cycle1, (2) strands that experienced both a lead at position 1 and a lag atposition 2 during cycle 1, and (3) strands that experienced a lag atposition 1 during cycle 1 along with a subsequent lead at position 1during cycle 2. A portion of these strands will also experience a cycle2 lead to the third position, however, since they have “caught up” tothe other strands with available sites at the second position, they arelumped together with them and further leads are considered as portionsof the combined population.

Similar events occur at each template position during cycle 2. Theevents of cycle 3 (green shades) follow very similar patterns to thosedescribed for cycle 2.

h. Mathematical Models of Dephasing and Rephasing

We may derive general equations that describe all the incorporationevents at each position and for each cycle. If we denote the relativemagnitude (out of 1) of the number of strands that remain unincorporatedfor position i at the end of a cycle j as R_(i,j), and the number ofstrands that are available for incorporation in the next cycle asA_(i,j) then

R_(i,j)=R_(i,j-1)−A_(i,j-1)(1−G_(i))  (1) and

A_(i,j)=R_(i,j)−R_(i-1,j)+A_(i-1,j)D_(i-1)  (2)

To explain the derivation of Equation 1, we use the example in FIG. 13,which shows a portion of the chart from FIG. 11. Only cycles 2 and 3 areshown for position 2. R_(2,3) is comprised of R_(2,2) minus a portion(1-G₂) of A_(2,2).

To explain the derivation of Equation 2, we use the example in FIG. 14,which shows a portion of the chart from FIG. 11. Only cycle 3 is shownfor positions 2 and 3. A_(3,3) is comprised of R_(3,3) minus R_(2,3)plus the lead portion (D₂) of A_(2,3).

It should be noted that for any particular cycle and position, thenumber of available strands, A_(i,j), is generally fewer than the numberof remaining strands, R_(i,j), since some templates at the particularposition are still lagging and unavailable, but may “catch up” in futurecycles.

The signal that is produced, S_(i,j), at the i^(th) position at the endof the j^(th) cycle comes from the proportion of the strands that areavailable, A_(i,j), that undergo event T-I

S _(i,j) =A _(i,j)(1−D _(i) −G _(i))  (3).

In one embodiment, the model is used to apply the lead-lag compensationbased on calibration of parameters, before or during the test, and toprovide an initial estimate of the base identity at each location asdetermined during the sequencing run. In a particular embodiment, G_(i)and D_(i) for each nucleotide may be pre-calibrated or measured duringthe sequencing procedure. In general, the model is constructed with lagparameters that are applied to each cycle and lead parameters that arerecursively applied to each cycle.

In a particular embodiment, the lead-lag matrix is formulated after aninitial draft sequence is measured. This allows application of theproper set of G_(i) and D_(i) parameters to each cycle based on thenucleotides identified at each position in the draft sequence. In afurther embodiment, the re-phasing of data is iterated using the resultof one re-phasing calculation to select an updated set of G_(i) andD_(i) parameters for the next iteration.

i. Simulated Dephased Sequencing Data

We may use the relationships derived in the previous section to generatesequence data that simulate the signals that might occur when portionsof every incorporation undergo a lead and a lag. As an example, we usethe lead and lag factors below and generate simulated sequence dataassuming the maximum signal from the template is 10,000 counts and thetemplate has a 35 base repeating sequence of AGCTAGCTAGCT. FIG. 15 showsdata generated from the iterative application of equations 1-3 using theparameters in Table 1.

TABLE 1 Lead Factors and Lag Factors for Nucleotides A G C T LeadFactors 4.10% 4.20% 4.30% 4.40% Lag Factors 1.10% 1.20% 1.30% 1.40%

FIG. 15 shows that with the presence of a lead and lag component, thereis a cumulative effect that reduces the signal from the expectednucleotide at a particular cycle and “spreads” some of the signalforwards and backwards. As the number of cycles increases, it becomesmore and more difficult to directly read the correct base from thegraph, thereby limiting the effective read length of the template.

ii. Re-Phasing Sequencing Data

Data herein (Example 11, FIGS. 33-34) demonstrate that applying themethods and the below described equations of the invention, exemplary16-base and 25-base nucleotide sequences were sequenced with highfidelity. The high quality of the data, particularly in the last severalbases in FIG. 34, demonstrates that the read length will not be limitedby signal decline. Thus, it is contemplated that the invention's methodsare applicable to sequences containing at least 16 nucloetides, at least25 nucleotides, at least 35 nucleotides, at least 50 nucleotides, atleast 100 nucleotides, at least 1,000 nucleotides, and at least 10,000nucleotides. Further description of the equations used to re-phasesequencing data is described as follows.

We constructed a matrix equation that describes a model for the reducedmeasured signal, I_(Mi), from the lead and lag effect in the i^(th)cycle from the original template populations, I_(Ai), for all cycles,i=1 to N. Here each of the intensity matrices ([I_(Ai)] and [I_(Mi)])have N rows (one for each cycle) and four columns (one for each color).

$\begin{matrix}{{\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{MN}\end{bmatrix} = {K_{{Lead}/{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{AN}\end{bmatrix}}},} & (4)\end{matrix}$

where the lead/lag matrix, K_(Lead/Lag), is a square N×N matrix of thefollowing form:

$\begin{matrix}{K_{{Lead}/{Lag}} = {\quad\begin{bmatrix}R_{{{Lag}/{Lead}},1} & R_{{{+ 1}\; {Lead}},1} & R_{{{+ 2}{Lead}},1} & R_{{{+ 3}{Lead}},1} & \ldots & R_{{{+ {({N - 1})}}{Lead}},1} \\R_{{{- 1}{Lag}},2} & R_{{{Lag}/{Lead}},2} & R_{{{+ 1}{Lead}},2} & R_{{{+ 2}{Lead}},2} & \ldots & R_{{{+ {({N - 2})}}{Lead}},2} \\R_{{{- 2}{Lag}},2} & R_{{{- 1}{Lag}},3} & R_{{{Lag}.{Lead}},3} & R_{{{+ 1}{Lead}},3} & \ldots & R_{{{+ {({N - 3})}}{Lead}},3} \\R_{{{- 3}{Lag}},4} & R_{{{- 2}{Lag}},4} & R_{{{- 1}{Lag}},4} & R_{{{Lag}/{Lead}},4} & \ldots & R_{{{+ {({N - 4})}}{Lead}},4} \\\vdots & \vdots & \vdots & \vdots & \ddots & \vdots \\R_{{{- {({N - 1})}}{Lag}},N} & R_{{{- {({N - 2})}}{Lag}},N} & R_{{{- {({N - 3})}}{Lag}},N} & R_{{{- {({N - 4})}}{Lag}},N} & \ldots & R_{{{Lag}/{Lead}},N}\end{bmatrix}}} & (5)\end{matrix}$

The diagonal terms, R_(Lag/Lead,i), in the K_(Lead/Lag) matrix above isthe fractional remaining signal in the i^(th) cycle from the i^(th)position of the templates after all of the leads and lags to that point.Each of the terms in the upper triangular portion of the matrix,R_(−kLead,i), is the fractional contribution to the signal in the i^(th)cycle from k positions forward of the i^(th) position. Each of the termsin the lower triangular portion of the matrix, R_(−kLag,i), is thefractional contribution to the signal in the i^(th) cycle from kpositions before the i^(th) position. In most systems, the terms with kgreater than about 4 (5 positions or more away from the positioncorresponding to the cycle number) are negligible. The diagonal termsare close to 1 for the earlier cycles and do not drop below about 0.25for the later cycles. Thus, this matrix is invertable.

In order to compensate for both sequence leads and lags, we solved for[I_(Ai)] in Equation 4 by taking the inverse, K_(Lead/Lag) ⁻¹, ofK_(Lead/Lag) (Equation 5) to get an estimate of the actual templatepopulation [I_(Ai)]:

$\begin{matrix}{\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{AN}\end{bmatrix} = {{K_{{Lead}/{Lag}}^{- 1}\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{MN}\end{bmatrix}}.}} & (6)\end{matrix}$

When the lead rates for all the nucleotides are identical and the lagrates for all of the nucleotides are identical, then the lead/lagmatrix, K_(Lead/Lag), does not depend on the sequence. This makesEquation 6 linear and the inverse of the matrix is deterministic. Inthis case the inverse of the lead/lag model gives the correction matrix,K_(Lead/Lag) ⁻¹, which is applied just once at the end of a run andtakes into account all of the signals from the first to the last base.

If on the other hand the lead and lag factors vary from one nucleotideto the next, then the lead/lag matrix, K_(Lead/Lag), depends on theactual sequence (solution of [I_(Ai)] in Equation 6) and the problem isnon linear. In other words, we need to determine an estimate for thetrue value of the intensities of each base in the sequence when thegoverning equations depend on this solution.

To solve the non-linear problem, one can estimate a solution and iterateuntil the solutions converge. We may use the raw out-of-phase sequencedata to make an initial estimate of the sequence by taking the maximumvalue at each cycle, using this information to determine the lead andlag rates for each position, construct a lead/lag matrix (K_(Lend/Lag)),take the inverse of that matrix and solve for the corrected, re-phasedsequence. We can then use the new sequence to make a new estimate of thelead/lag matrix, etc. As long as the various lead and lag factors arefairly close to one another, this method should converge in about two orthree iterations.

iii. Additional Factors

The above method is a very powerful way of “cleaning up” sequence datathat has been dephased due to the lead and lag phenomena. The matrixcondition number determines when matrix manipulations will be sensitiveto small numerical variations. A matrix condition near 1 means thematrix is well behaved, while a large condition number means the matrixis ill-conditioned or sensitive to small numerical inaccuracies.

FIG. 16A shows a simulated 35 base read data with 10% noise added andlead and lag factors of 1% each. FIG. 16B shows an accurate lead/lagcompensation reconstruction using the inverse lead/lag matrix, K⁻¹_(Lead/Lag). The condition number for this lead/lag matrix is 5. FIG.17A shows the same exemplary 35 base sequence with 10% noise and a leadand lag factor of 1.75% each and FIG. 17B shows the reconstruction.Here, the condition number is 550 and reconstruction is poor. In orderto have a fairly precise reconstruction of the data, lead/lag matricesdesirably have a condition below about 20. FIG. 18 plots the matrixcondition numbers below 20 for 35 base lead/lag matrices with variousvalues of lead and lag. Because the lead/lag matrix is calculatedindependent of the DNA sequence in a template for the case wherenucleotides all have equal lead ratios and equal lag ratios, we are ableto determine our ability to accurately call bases without considerationof the A, C, T, and G content of the templates.

Even if a 35 base lead/lag matrix is ill-conditioned and produces poorreconstruction, smaller matrices from a portion of the same data maystill be well behaved. For example, FIG. 17C shows the reconstruction ofthe first 18 bases for a lead and lag of 1.75% each (same conditionsthat produced the poor reconstruction for the 35 base read). Here the18×18 lead/lag matrix has a condition number of 3.7. The matrix becomesill-conditioning when cumulative contributions from the lead or lag orboth generate signals that are on the order of the signal from the truebase (where the position equals the cycle number).

The above shows that changing the read length can provide an accuratereconstruction of earlier portions of the data. Thus, we can plot thelead and lag factors that will cause matrices of different sizes (readlengths) to have a condition number of 20 (the point where matricesbecome too ill-conditioned for precise reconstruction). For example, ifa 23×23 matrix produces a condition number of 20, then we would restrictthe read length to a maximum of 23 as reconstruction using the 24th base(as well as any additional bases) would likely create a matrix that istoo ill-conditioned to accurately reconstruct the data. FIG. 19 showsthe read lengths for various lead and lag factors. This plot provides amethod for predicting the read length obtainable from a sequencingsystem based on two factors: the purity of the nucleotides (majorcontributor to the lead) and the polymerase incorporation efficiency(major contributor to the lag). This result also shows that if both thelead and lag factors are below about 0.5%, this results inreconstruction of a 100-base read.

F. Field Flattening

In one embodiment, when a chip of uniform dye concentration is imaged,it may be desirable that all of the pixels in the resultant image havenearly the same intensity, with variations reflecting only therelatively small distribution inherent in the camera's optical system.In practice, however, the inventors have found that variations inillumination and filter response produce a significant spatiallycorrelated pattern in an image. The inventors have also found that thepattern is highly reproducible and has a linear response to changes indye concentration and camera exposure. These conditions lead to thefollowing algorithm for removing this spatial correlation between pixelintensity and location of the pixel on the solid substrate.

First, for each machine and each filter, we image the pattern of aspatially uniform fluorescence on a dyed microscope slide. Second, theimage is smoothed using a low-pass filter. In the resultant smoothedimage, M, we choose an origin point, M_(x0,y0). The choice of the pointis fairly arbitrary but it is selected from a region in which thesmoothed images of all of the filters have low intensity gradients tominimize the impact of changes in the system. Third, the intensity ofeach pixel at a point in a raw image (R_(x,y)) is replaced by itsfield-flattening value, F_(x,y), where F_(x,y)=R_(x,y)M_(x0,y0)/M_(x,y)and M_(x,y) is the value of the model image at the same spatial locationas the raw image pixel. The resultant “Field Flattened” image, F, hasintensities that are now solely dependent on the camera exposure and dyeconcentration, and do not have any correlation with the spatial locationof the pixel in the image.

The invention's algorithms and equations for field flattening aredistinguished from those described by, for example Eltoukhy et al.(2006), since the algorithms of Eltoukhy et al. relate to signal noisethat is uncollrelated to system parameters (e.g., uneven light source).In contrast, the signal noise that is corrected by the invention'smethods is correlated to the signal's position across the solidsubstrate. In one embodiment, each pixel is corrected (i.e., fieldflattened) based on a previously calibrated baseline intensity at thatpixel position and a scaling factor based on for example a longerexposure time.

G. Spot Location in the Array

In one embodiment, the present invention contemplates a processing step(preferably in a series of processing steps as discussed above) forlocating the spots in the array. In one embodiment, the spot locatingimage processing algorithm uses the fact that the spots on the chip arein a regular hexagonal pattern along vertical columns and diagonal rows.To find the columns of spots, image pixel values are summed along thevertical direction. This results in a one-dimensional set of data thatresembles a sinusoidal pattern. The peaks of the pattern are measuredand used to determine the period and phase of the pattern. There arethen used to guide a search to determine a set of equations for verticallines that approximately bisect each of the spots in a column. Theresult is a set of equations for parallel lines (slope, intercept andspacing) at regular spacing. A number of these lines are then probed toestablish a second sinusoidal-like pattern of intensities along thelines. These are then used to determine the period, angle and phase ofthe diagonal lines that bisect each of the spots. These second set oflines are at a 60 degree angle from the vertical lines. The intersectionof the diagonal set of lines and the vertical set of lines give anestimate for the subpixel locations of each of the spot centers.

H. Image Sharpening

In one embodiment, the present invention contemplates a processing step(preferably in a series of processing steps as discussed above) tosharpen the image of the spots on the array. This is particularly usefulif chips are constructed with tightly packed spots. In such a case, itmight be beneficial to run the images through a sharpening filter inorder to reduce the amount of blur or spread for each of the spots. Thiswill reduce the amount of light energy blooming into adjacent spots.Similarly, if the optics for the system cannot sufficiently resolvespots on the chip, then the application of a sharpening filter may alsohelp to precisely analyze the images. A number of sharpening algorithmsmay be used to narrow the spread of the spots. One embodiment uses aWiener filter (as described, for example, in The Image ProcessingHandbook, by John C. Russ, Published by CRC Press, 2006, ISBN0849372542, 9780849372544, 817 pages) to make the diameter of the spotssmaller and remove light energy from adjacent spots.

I. Spot Brightness Determination

In one embodiment, the present invention contemplates a processing step(preferably in a series of processing steps as discussed above) todetermine spot brightness. In one embodiment, the pixels that surroundeach of the subpixel locations of the spots are summed to determine anestimate for the spot brightness. The local set of pixels that isselected depends on both the diameter of the spots and the locationwithin the pixel of the subpixel center location. For example, if thesubpixel location is close to the top of the pixel that contains thecenter, then more pixels above the pixel that contains the center arecounted than pixels below the pixel containing the center.

In one embodiment, the above method for making spot brightnessmeasurements is repeated independently for each of the four differentcolor channels (four separate images) and the sharpening and neighborinfluence (see below) correction calculations are applied, then thecolor crosstalk correction is applied (see below). In one embodiment,the result of the color crosstalk calculation produces a list of fourvalues (one for each dye color) for each spot in the images that may beused to call the base for that sequencing cycle.

J. Neighbor Influence Elimination

In addition to each of the spots expanding beyond its physical bounds,the light from one spot (bead) may illuminate an adjacent spot and makeit appear to have more of the color of its neighbors. This might happenbecause light being emitted from one bead make impinge on an adjacentbead, be reflected within the bead and then reemitted from that bead.This neighbor influence may be eliminated by, in one embodiment,constructing an influence or “spreading” matrix, then applying theinverse of this matrix to the data. To formulate the solution to theneighbor influence problem from spot data that is in hexagonal form, itis convenient to first put the data into a rectilinear array. This isdone by shifting the even vertical columns up by ½ of a pixel as shownbelow. Thus, a two-dimensional rectilinear matrix, whose elementsrepresent the magnitude of each spot in the original image of thehexagonal array of spots, may be used. To further facilitate matrixmanipulations, the rectilinear spot matrix may be made into a spotvector by stacking the columns from the two-dimensional matrix under oneanother to form a one dimensional array or vector. In other words, thesecond column is appended to the bottom of the first, the third to thebottom of the second, etc., thereby generating a 1×N² vector formed froman N×N spot matrix. In one embodiment, a “spreading matrix” is nextformed that represents the magnitude of the influence from a spot toneighboring spots. In a general formulation, a central spot may bethought of as influencing the nearest six neighbors surrounding thecentral spot by a fraction, A, of the central spot brightness, the nextnearest neighbors by a smaller fraction, B, etc. If the central spot isvery bright, then its neighbors may appear to be emitting their ownlight of the same color as the central spot, even if they actuallygenerate none of their own light in that color. The spreading matrix isformulated such that if it is applied to an ideal image of singleelement spots (each spot is an idealized point and does not extendbeyond one element of the matrix), then the resultant matrix will havespots that have been spread across several elements due to the neighborinfluence phenomenon. Thus, the spreading matrix is a model for theinfluence of any spot in the image to any other spot in the image.

For hexagonal arrayed spots that have been make into a one-dimensionalvector with dimensions N²×1 (in other words, a concatenation of all thecolumns of the matrix), the spreading matrix, S, may be formulated as aN²×N² matrix. An example 25×25 spreading matrix corresponding to a 5×5spot image that has the three levels of neighbor influence (A for theclosest 6 neighbors, B for the next closest 6 and C for the thirdclosest 6) is shown below.

If the spreading matrix, S, is inverted, S⁻¹, it may be used toeliminate the neighbor influence modeled by the spreading matrix. If wemultiply the measured spot matrix (in the form of a vector), ν_(spot),by the inverse of the spreading matrix, S⁻¹, we can generate an estimatefor the spot matrix with the neighbor influence removed,ν_(uninfluenced)

ν_(uninfluenced)=S⁻¹ν_(spot).

The method described above for removing the neighbor influence cangenerate sizable spreading matrices on the order of N⁴ and therefore maybe computationally intensive for typical images. Since the influence ofspots that are relatively far from the spot of interest have relativelynegligible influence, it is possible to reduce the size of the spreadingmatrices used for the calculation and perform the calculation on smallersubsections of the image at a time. This can significantly reduce thecomputational complexity and computer memory requirements for thecalculation. It should be understood that the methods set forth abovemay be generalized in algorithms that are more efficient or operate onsmaller portions of the image.

K. Spectral Crosstalk Calibration

In one embodiment, it may be desirable to correct the data to accountfor color crosstalk. This may be done using methods known in the art(e.g., U.S. Pat. No. 7,209,836 incorporated by reference) as well asmethods disclosed herein (see Example 10, FIG. 32). For example, afour-color fluorescent detection system (for detection of the exemplarycolors blue, green, yellow and red) has one detector channel for each ofthe four different color dyes. However, because the dyes have fairlybroad spectra, there is some detection of dyes in adjacent colorchannels. For example, when exciting only a green dye, a signal isvisible in the yellow channel as well as the green channel. If thisspectral crosstalk is calibrated, it may be removed from subsequentmeasurements even if the dyes are mixed in unknown quantities. Todetermine actual fluorescent intensities for the four colors, A, B, Cand D from measured detector outputs, M_(A), M_(B), M_(c), M_(D) incorresponding channels, one needs to know all of the spectral crosstalkfactors: R_(AB), R_(BA), R_(BC), R_(CB), R_(CD), and R_(DC). Forexample, R_(AB) is the ratio between the portion of the signal in the Achannel coming from the B dye and the actual intensity of the B dye. Iffor instance R_(AB) is 20%, then the A channel will have an additionalsignal equal to 0.2 times the actual B dye intensity in the B channel.Thus for channel B, the observed measurement, M_(B), is the directmeasurement of B and the two contributions from the adjacent channels(if any): M_(B)=B+R_(BA)A+R_(BC)C (6). For the four channels, this maybe written in matrix form:

$\begin{matrix}{{\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix} = {K\begin{bmatrix}A \\B \\C \\D\end{bmatrix}}}{where}{K = \begin{bmatrix}1 & R_{AB} & 0 & 0 \\R_{BA} & 1 & R_{BC} & 0 \\0 & R_{CB} & 1 & R_{CD} \\0 & 0 & R_{DC} & 1\end{bmatrix}}} & (7)\end{matrix}$

Each of the six spectral crosstalk factors may be determined through anexperiment with pure dyes. We want to solve for the actual fluorescentsignals, A, B, C and D given the detector measurements, M_(A), M_(B),M_(C), M_(D). Thus, we want to solve the above matrix Equation (7). Thisis equation (8):

$\underset{\ldots}{\begin{bmatrix}A \\B \\C \\D\end{bmatrix}} = {K^{- 1}\underset{\ldots}{\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix}}}$

where K⁻¹ is the inverse of matrix K. Although the inverse of matrix Kmay be written out in terms of the six spectral crosstalk factors, it issomewhat complex and is best performed by plugging in the numbers andletting the computer take the inverse. The results are discussed inExample 10 which demonstrate that a base in the sequence would have beenmiscalled were the spectral crosstalk calibration not performed.

Any multicolor sequencing by synthesis device may be calibrated usingthe above equations and the resultant spectral crosstalk matrix may beused in all four color measurements from the device. In one embodiment,if we also include information on the relative magnitude of each of thefour colors, then we can also correct for differences in perceived dyebrightness from one channel to the next. Multiplying the matrix K by adiagonal matrix, whose diagonal terms are the relative brightness foreach dye, produces a new matrix K whose inverse will automatically scalethe dyes to be consistent with one another.

K. Base Calls

As discussed above, at each sequencing by synthesis cycle, the signalsthat are observed in the four color channels are used to both determinethe most likely base at that cycle (base call) and to determine aquality score for the base call. Because of a number of factors, it maynot always be the case that the brightest color in the raw dataindicates the most likely base. Thus, it may be desirable to correct forat least one, and more particularly all, of the following phenomena thatwere discussed supra: field flattening, spectral crosstalk, sequencelead and sequence lag. After the correction factors (field flattening,spectral crosstalk and/or lead-lag compensation) have been applied, abase is called based on the maximum signal between the four channels.The output of the base calls may be a file similar to a FASTA format. Inone embodiment, this file is also accompanied by a quality score file.

To optimize the alignment and assembly of the data into contigs, it isdesirable to have a precise quality score associated with each basecall. A quality file may be generated that encodes quality scores foreach cycle. Preserving the information for all four bases is alsodesirable to allow the sequence alignment software to examine severallikely calls instead of only the one with the highest signal.

M. Software Appendices A-C

The below software Appendices A, B and C (copyright IntelligentBio-Systems, Inc., 34 Bear Hill Road, Waltham, Mass. 02451) providesource code for implementing the present invention. Appendix A is asource code for correcting a raw image using a flat map calibrationimage, as exemplified by the code under FlattenImageInArray andAdjustRawValue. Appendix B is a source code for applying the inversecross-talk array to four filter images. In particular, theFindBeadIntensities method calls ProcessOneBead to apply the spectralcrosstalk correction matrix. Appendix C is a source code for creating aflat map calibration image. In one embodiment, this is a process thatuses a combination of automated and manual steps. The automated stepsare exemplified by the emoveSpikesWithSlope and LocalSmoothing methods.The manual steps are exemplified by ImageJ to replace spikes in thecalibration image with smoothed data. The manual and automated steps maybe carried out in any order. In a particular embodiment, the manualsteps are carried out before the automated steps.

EXPERIMENTAL

The following examples serve to illustrate certain exemplary embodimentsand aspects of the present invention and are not to be construed aslimiting the scope thereof.

Example 1 Materials And Methods

The following is a brief description of the exemplary materials andmethods used in the following Examples. All solvents and reagents werereagent grades, purchased commercially and used without furtherpurification. Protected nucleosides5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine,N⁴-benzoyl-5′-O-tert-butyldimethylsilyl-2′-deoxycytidine,N⁶-Benzoyl-5′-O-tert-butyldimethylsilyl-2′-deoxyadenosine,N²-isobutyryl-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine werepurchased from CNH Technologies, Inc. All other chemicals were purchasedfrom Sigma-Aldrich.

Example 2 Synthesis of 3′-O-Azidomethyl Nucleotides

The synthesis of 3′-O-azidomethyl-dNPTs is described in FIG. 20.Briefly, reaction of 5′-O-TBDMS-2′-deoxynucleosides (5) with a mixtureof DMSO, acetic acid, and acetic anhydride installed the3′-O-methylthiomethyl group (3′-O-MTM, 6), which upon treatment withSO₂Cl₂ converted to activated 3′-O—CH₂Cl (7). The latter can bemonitored in TLC as 3′-OH (5) after dissolving in wet organic solventdue to fast hydrolysis of the —CH₂Cl group. The3′-O—CH₂Cl-2′-deoxynucleoside (7) is then treated with NaN₃ in dry DMFwithout purification to convert to 3′-O-CH₂N₃ (8).3′-O-azidomethyl-2′-deoxynucleosides of A, T, and C (9a-9c) wereobtained in good yield after deprotection of the 5′-O-TBDMS group asdescribed in the FIG. 20. Similar synthesis route for guanosine (G, 9d),lead only very low yield (>10%) due to formation of a number of sidereaction products. To circumvent this, a new method was introduced forthe synthesis of guanosine analog (14) which is described in the FIG.21, which involved protection of the O⁶-group by diphenycarbamoyl group.After protection of this particular group, the intermediate (12-14)became less polar, making easier to purify, and lead good overall yieldin the azidomethyl group installation step.

Example 3 Synthesis of N⁶-benzoyl-3′-O-(azidomethyl)-dA (9a)

The following describes exemplary synthesis steps for compounds shown inFIG. 20.

A. Synthesis ofN⁶-Benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyadenosine(6a)

3.0 g N⁶-Benzoyl-5′-O-tent-butyldimethylsilyl-2′-deoxyadenosine (5a)(6.38 mmol) was dissolved in a mixture consisting of 11.96 mL DMSO, 5.46mL acetic acid, and 17.55 mL acetic anhydride and stirred at roomtemperature for 48 h. The reaction mixture was then neutralized treatingwith a sufficient amount of saturated NaHCO₃ solution and extracted withCH₂Cl₂ (3×100 mL). The combined organic extract was then washed with asaturated NaHCO₃ solution (100 mL), dried over Na₂SO₄, and concentratedunder vacuum. The resultant yellowish oil was then purified on silicagel column (Hex: EtOAc/1:1 to 1:4) to obtain the productN⁶-benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyadenosine(6a) as white powder in 71% yield (2.4 g, R_(f) 0.6, EtOAc: hex/7:3).HR-MS: obs. m/z 530.2273, calcd. for C₂₅H₃₆O₄N₅SiS 530.2257 [M+H]⁺.¹H-NMR (CDCl₃): δ_(H) 9.00 (s, 1H), 8.83 (s, 1H), 8.35 (s, 1H), 8.05 (d,J=7.6 Hz, 2H), 7.62 (m, 1H), 7.55 (m, 2H), 6.55 (t, J=7.19 Hz, 1H), 4.73(m, 2H), 4.68 (m, 1H), 4.24 (m, 1H), 3.88 (dd, J=11.19, 3.19 Hz, 1H),2.74-2.66 (m, 2H), 2.35 (s, 3H), 0.94 (s, 9H) and 0.13 (s, 6H) ppm.

B. Synthesis of N⁶-benzoyl-3′-O-(azidomethyl)-2′-deoxyadenosine (9a)

To 0.4 gN⁶-benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyadenosine(0.76 mmol) dissolved in 7 mL dry CH₂Cl₂ was treated with 0.4 mLcyclohexene and 155 μL SO₂Cl₂ (1.91 mmol) at 0° C. for 2 h. During thistime the starting material completely converted to 7a which was shown bydisappearance of the starting material and appearance of 3′-OH analog(5a) in TLC (EtOAC:Hex/7:3, R_(f)˜0.3; the 3-CH₂Cl (7a) could notdetected in TLC due to decomposition in TLC plate to 5a). Then solventwas removed by rotary evaporation and kept about 10 minutes in highvacuum pump. Then dissolved in 5 mL dry DMF and treated with 400 mg NaN₃(6.6 mmol) at room temperature for 3 h. Then the reaction mixture waspartitioned in H₂O/CH₂Cl₂, the combined organic part was dried overNa₂SO₄ and concentrated by rotary evaporation. The crude sample was thendissolved in 5 mL MeOH and treated with 300 mg NH₄F (8.1 mmol) more than38 h. Then MeOH was removed by rotary evaporation. After partioning inH₂O/EtOAc, the combined organic part was dried over Na₂SO₄,concentrated, and purified by silica gel column chromatography (100%EtOAc to 98:2, EtOAc/MeOH) resulting 150 mg of 9a as white powder (48%yield in three steps). HR-MS: Obs. m/z 411.1530, calcd for C₁₈H₁₉O₄N₈411.1529 [M+H]⁺. ¹H-NMR (CDC₃): δ_(H) 8.84 (brs, 1H), 8.70 (brs, 1H),8.08 (m, 1H), 7.76-7.54 (m, 5H), 6.47 (t, J=5.6 Hz, 1H), 4.83 (m, 2H),4.78 (m, 1H), 4.39 (m, 1H), 4.09 (d, J=12.78 Hz, H₅′, 1H), 3.88 (d,J=12.78 Hz, H₅″, 1H), 3.09 (m, H₂′, 1H), and 2.65 (m, H₂″, 1H) ppm.

Example 4 Synthesis of 3′-O-azidomethyl-dT (9b)

The following describes exemplary synthesis steps for compounds shown inFIG. 20.

A. Preparation of3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine(6b)

2.0 g 5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine (5b) (5.6 mmol)was dissolved in a mixture consisting of 10.5 mL DMSO, 4.8 mL aceticacid, and 15.4 mL acetic anhydride and stirred for 48 h at roomtemperature. The mixture was then quenched by treating with a saturatedNaHCO₃ solution and extracted with EtOAc (3×100 mL). The combinedorganic extract was then washed with a saturated solution of NaHCO₃ anddried over Na₂SO₄, concentrated under vacuum, and finally purified bysilica gel column chromatography (Hex: EtOAc/7:3 to 1:1). The3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine(6b) was obtained as white powder in 75% yield (1.75 g, R_(f)=0.6, hex:EtOAc/1:1). HR-MS: Obs. m/z 417.1890, cald. for C₁₈H₃₃N₂O₅SSi 417.1879[M+H]⁺. ¹H-NMR (CDCl₃): δ_(H) 8.16 (s, 1H), 7.48 (s, 1H), 6.28 (m, 1H),4.62 (m, 2H), 4.46 (m, 1H), 4.10 (m, 1H), 3.78-3.90 (m, 2H), 2.39 (m,1H), 2.14, 2.14 (s, 3H), 1.97 (m, 1H), 1.92 (s, 3H), 0.93 (s, 9H), and0.13 (s, 3H) ppm.

B. Preparation of 3′-O-(azidomethyl)-2′-deoxythymidine (9b)

To 1.095 g3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine(6b) (2.6 mmol) dissolved in 10 mL dry CH₂Cl₂ were added 1.33 mLcyclohexene and 284 μL SO₂Cl₂ (3.5 mmol) at 0° C. and stirred at theice-cold temperature for 1.5 h. Then the flask temperature was broughtto room temperature and transferred to a round bottom flask. Thevolatiles were removed by rotary evaporation followed by high vacuumpump. Then the crude sample was dissolved in 5 mL dry DMF and 926 mgNaN₃ (15.4 mmol) was added to it and stirred for 3 h at roomtemperature. The crude sample was dispersed in 50 mL distilled water andextracted with CH₂Cl₂ (3×50 mL), the organic extracts were combined anddried over Na₂SO₄ and concentrated by rotary evaporation. The crudesample was then dissolved in MeOH (5 mL) and treated with NH₄F (600 mg,16.2 mmol) for 24 h at room temperature. Then reaction mixture wasconcentrated and partitioned between H₂O/CH₂Cl₂ and the combined organicextract was dried over Na₂SO₄, concentrated, and purified the product bysilica gel column chromatography using Hex: EtOAc/1:1 to 2:5 resultingthe final product (9b) as white powders 550 mg, 71% yield in threesteps, R_(f)=0.3, Hex: EtOAc/1: 1.5). HR-MS: Observed m/z 298.1146,calcd for C₁₁H₁₆O₅N₅ 298.1151 [M+H]⁺. ¹H-NMR (CDC₃): δ_(H) 8.30 (brs,1H), 7.40 (s, 1H), 6.14 (t, J=6.8 Hz, 1H), 4.79-4.70 (m, 2H), 4.50 (m,1H), 4.16 (m, 1H), 4.01-3.84 (m, 2H), 2.45 (m, 2H) and 1.95 (s, 3H) ppm.

Example 5 Synthesis of N⁴-Benzoyl-3′-O-(azidomethyl)-dC (9c)

The following describes exemplary synthesis steps for compounds shown inFIG. 20.

A. Preparation ofN⁴-Benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxycytidine(6c)

3.5 g N⁴-benzoyl-5′-O-tert-butyldimethylsilyl-2′-deoxycytidine (5c)(7.65 mmol) was dissolved in a mixture consisting of 14.7 mL DMSO, 6.7mL acetic acid, and 21.59 mL acetic anhydride and stirred for 48 h atroom temperature. During this period of time, a complete conversion toproduct was observed by TLC (R_(f)=0.4, EtOAc:hex/10:1). The mixture wasthen neutralized with a saturated NaHCO₃ solution and extracted withCH₂Cl₂ (3×100 mL). The combined organic extract was then washed withsaturated solution of NaHCO₃ and dried over Na₂SO₄, and concentratedunder vacuum. The product was then purified by silica gel columnchromatography (EtOAc: hex./2:1 to 9:1) to obtainN⁴-benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxycytidine(6c) as white powder in 73% yield (2.9 g, R_(f)=0.6, EtOAc:hex/9:1).HR-MS: obs. m/z 506.2134, cald. for C₂₄H₃₆O₅N₃SiS [M+H]⁺. 506.2145.¹H-NMR (CDCl₃): δ_(H) 8.43 (d, J=7.1 Hz, 1H), 7.93 (m, 2H), 7.64 (m,1H), 7.54 (m, 3H), 6.30 (m, 1H), 4.62 & 4.70 (2×d, J=11.59 Hz, 2H), 4.50(m, 1H), 4.19 (m, 1H), 3.84 & 3.99 (2×dd, J=11.59 & 2.79 Hz, 2H), 2.72(m, 1H), 2.21 (m, 1H), 2.14 (s, 3H), 0.99 (s, 9H), and 0.16 (s, 6H) ppm.

B. Preparation of N⁴-Benzoyl-3′-O-(azidomethyl)-2′-deoxycytidine (9c).To 0.5580 gN⁴-benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxycytidine(6c)

(1.04 mmol) dissolved in 8 mL dry CH₂Cl₂ were added 0.56 mL cyclohaxeneand 220 μL SO₂Cl₂ (2.7 mmol) at 0° C. and stirred at the ice-coldtemperature for 1 h. During this time, the starting material convertedto the chlorinated product as shown by the 3′-OH (5c) compound in theTLC. The volatiles were then removed under vacuum and resuspended in dryDMF (5 mL) and treated with NaN₃ (400 mg, 6.6 mmol) and stirred for 2 hat room temperature. The sample was then partitioned between water andCH₂Cl₂ and the organic extracts were combined and dried over Na₂SO₄ andconcentrated under vacuum. The crude sample was then dissolved in MeOH(5 mL) and treated with NH₄F (600 mg, 16.2 mmol) for 20 h at roomtemperature. Then solvent was removed under vacuum and extracted withCH₂Cl₂ and the organic extract was then dried over Na₂SO₄ andconcentrated under vacuum. The sample was then purified by silica gelcolumn chromatography (Hex:EtOAc 1:4 to 1:10), and the product (9c) wasobtained as white powdery substance (˜200 mg, 50% yield in three steps,R_(f)=0.5, EtOAc:Hex/5: 0.5). HR-MS: Obs. m/z 387.1408, calcd forC₁₇H₁₉O₅N₆ 387.1417 [M+H]⁺. ¹H-NMR (CDC₃): δ_(H) 8.30 (d, J=7.2 Hz, 1H),7.93 (d, J=7.50 Hz, 1H), 7.66-7.51 (m, 5H), 6.18 (t, J=6.4 Hz, 1H),4.81-4.68 (m, 2H), 4.52 (m, 1H), 4.25 (m, 1H), 4.08-3.88 (m, 2H), 2.69(m, 1H), and 2.50 (m, 2H) ppm.

Example 6 Synthesis ofN²-isobutyryl-O⁶-diphenylcarbamoyl-3′-O-azidomethyl-dG (14)

The following describes exemplary synthesis steps for compounds shown inFIG. 21.

A. Preparation ofN²-isobutyryl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine(11)

5 g of N²-isobutyryl-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine(11.0 mmol) dissolved in 21 mL dry DMSO was treated with 10 mL aceticacid and 32 mL acetic anhydride, and stirred for 48 h at roomtemperature. The crude reaction mixture was then neutralized by adding aK₂CO₃ solution, and extracted with ethyl acetate (100×3 mL). Thecombined organic extract was then washed with saturated NaHCO₃ solution,dried over Na₂SO₄ and concentrated under vacuum. Then reaction mixturewas purified by a silica gel column chromatography resulting the product11 as white powder (3.9 g, 69% yield; R_(f)=0.35, CH₂Cl₂:MeOH/20:1).HR-MS: Obs. m/z 512.2344 cald. for C₂₂H₃₈O₅N₅SiS 512.2363 [M+H]⁺. ¹H-NMR(CDCl₃): δ_(H) 12.0 (s, 1H), 8.95 (brs, 1H), 8.09 (s, 1H), 6.24 (t,J=6.8 Hz, 1H), 4.73 (m, 2H), 4.66 (m, 1H), 4.16 (m, 1H), 3.81 (m, 2H),2.76 (m, 1H), 2.59 (m, 1H), 2.54 (m, 1H), 2.21 (s, 3H), 1.29 (m, 6H),0.91 (s, 9H), and 0.10 (s, 6H) ppm.

B. Synthesis ofN²-isobutyryl-O⁶-diphenylcarbamoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine(12)

To 1.0 gN²-isobutyryl-3′-O-(methylthimethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine(11, 1.95 mmol) dissolved in 22 mL dry pyridine were addeddiphenylcarbamoyl chloride (0.677 g, 2.92 mmol) and 1.02 mLN,N-diisopropylethylamine, and stirred at room temperature for 3 h undernitrogen atmosphere. The reaction mixture became dark red during thistime. The solvent was removed under high vacuum, and product was thenpurified by silica gel column chromatography using EtOAc:hex/1:1 to 7:3as mobile phase. The product 12 was isolated as yellowish powder (1.09g, ˜80% yield; R_(f)=0.7, EtOAc:hex (1:1)). HR-MS: Obs. m/z 707.3068calcd. for C₃₅H₄₇O₆N₆SiS 707.3047 [M+H]⁺. ¹H-NMR (CDCl₃): δ_(H) 8.25 (s,1H), 7.94 (brs, 1H), 7.47-7.37 (m, 10H), 6.42 (m, 1H), 4.75 (m, 2H),4.71 (m, 1H), 4.18 (m, 1H), 3.88-3.70 (m, 2H), 2.80 (m, 1H), 2.60 (m,1H), 2.19 (s, 3H), 1.30 (d, J=7.2 Hz, 6H), 0.93 (s, 9H) and 0.14 (s, 6H)ppm.

C. Preparation ofN²-isobutyryl-O⁶-diphenylcarbamoyl-3′-O-azidomethyl-2′deoxyguanosine(14)

To 786 mg 12 (1.1 mmol) dissolved in 8 mL dry CH₂Cl₂ was treated with0.56 mL cyclohexene and 180 μL SO₂Cl₂ (2.2 mmol) at 0° C. and stirredfor 1.5 h at the same temperature. The solvent was then removed byrotary evaporation, and further dried under high vacuum for 10 minutes.The crude product was then dissolved in 5 mL dry DMF and reacted with600 mg NaN₃ (10 mmol) at 0° C. and stirred at room temperature for 3 h.Reaction mixture was then partitioned H₂O/CH₂Cl₂, the combined organicextract was then dried over Na₂SO₄, and concentrated by rotaryevaporation. The crude was then dissolved in 5 mL dry MeOH, treated with500 mg NH₄F (13.5 mmol) at room temperature for more than 24 h. ThenMeOH solvent was removed by rotary evaporation, and partitioned(H₂O/CH₂Cl₂). The combined organic part was dried over Na₂SO₄ andconcentrated by rotary evaporation and purified by silica gel columnchromatography resulting pure product of 14 as white powder (230 mg,˜36% yield in three steps; hex:EtOAc 1:1 to 1:5, (R_(f)=˜0.3,Hex:EtOAc/1:4). HR-MS: Obs. m/z 588.2343, calcd for C₂₈H₃₀O₆N₉ 588.2319[M+H]⁺. ¹H-NMR (DFM-d₆): δ_(H) 8.64 (brs, 1H), 7.48-7.34 (m, 10H), 6.36(t, J=7.0 Hz), 4.93 (m, 2H), 4.76 (m, 1H), 4.04 (m, 1H), 3.57 (m, 1H),3.34 (m, 2H), 2.97 (m, 1H), 2.81 (m, 1H), and 1.10 (m, 6H).

Example 7 General Method for the Preparation of 3′-O-Azidomethyl-Dntps

The protected 3′-O-azidomethyl nucleoside (0.3 mmol) and proton sponge(75.8 mg; 0.35 mmol) were dried in a vacuum desiccator over P₂O₅overnight before dissolving in trimethyl phosphate (0.60 mL). Thenfreshly distilled POCl₃ (33 μL, 0.35 mmol) was added drop-wise at 0° C.and the mixture was stirred at 0° C. for 2 h. Subsequently, awell-vortexed mixture of tributylammonium pyrophosphate (552 mg) andtributylamine (0.55 mL; 2.31 mmol) in anhydrous DMF (2.33 mL) was addedin one portion at room temperature and stirred for 30 min. Triethylammonium bicarbonate solution (TEAB) (0.1 M, 15 mL, pH 8.0) was thenadded and the mixture was stirred for 1 h at room temperature. Then 15mL of NH₄OH was added and stirred overnight at room temperature. Theresulting mixture was concentrated in vacuo and the residue was dilutedwith 5 mL of water. The crude mixture was then purified with anionexchange chromatography on DEAE-Sephadex A-25 at 4° C. using a gradientof TEAB (pH 8.0; 0.1-1.0 M). Further purification by RP HPLC to givecorresponding target as colorless syrup:

Example 8 3′-O-Azidomethyl Nucleotides Cleavage

The 3′-O-azidomethyl group cleavage can be accomplished with a varietyof reducing agents such as phosphines. The cleavage agents that areparticularly desirable are those that are soluble in aqueous media anddo not cause any damage to the DNA. One particularly desirable agent istri(carboethoxy)phosphine (TCEP).

The 3′-O-azidomethyl nucleotides can be separated from nativenucleotides using RP HPLC. In the next experiment, the kinetics of the3′-O-azidomethyl TTP cleavage was studied. For this purpose, a 1 mMsolution of nucleotide was prepared in water and mixed with 50 mMsolution of TCEP/400 mM of Tris at pH 8.0 and incubated at 55 deg C. forvarious periods of time. After the incubation, the reaction was stoppedby mixing with 4 M NaOAc at pH=4.3 and an aliquot of reaction mixture(0.5 nmole of nucleotide) was injected and separated on the RP HPLCcolumn. The integrated peak area was then plotted against time.

Example 9 Sequencing by Synthesis Using 3′-O-Azidomethyl Nucleotides

We established conditions for sequencing by synthesis on the surfaceusing 3′-O-azidomethyl nucleotides. For this purpose we used variants ofthe 9 deg N polymerase that were developed specifically to incorporate3′-O-azidomethyl nucleotides. For these sequencing experiments we wereusing synthetic DNA templates that encompass self priming moieties.Examples of these DNA templates and their secondary structures are shownin FIG. 22.

These oligonucleotides carry a 5′-amino modification through which theyare attached to assay surface. The surface constitutes any surface thatis bio-compatible, has low fluorescent background and has functionalgroups on the surface which can be used to covalently attach the DNA. Inthe described case, pre-activated Codelink (from GE Healthcare) slideswere used for this purpose. The solution of the oligonucleotides (50 uM)for spotting was prepared in 150 mM phosphate/bicarbonate spottingbuffer (pH=7.5). The arrays were then spotted and incubated in the humidchamber at 25 deg C. overnight. After the incubation, the arrays wereblocked by washing in the 1×TBST/2% BSA buffer, rinsed with nucleasefree water and dried.

The sequencing was performed in a chambered slide (Grace Biolabs). Inthe experiment, each well was subjected to different number of cyclesusing the mixture of 3′-O-azidomethyl nucleotides with each extensioncycle followed by a cleavage cycle. Extension cycle consisted ofincubating the well with the solution containing: 3′-O-azidomethylnucleotide mix—75 uM, 9 deg N polymerase mutant—250 ug/ml, 10 mM KCl, 10mM (NH4)2SO4, 20 mM Tris-HCl, 4 mM MnSO4, 0.1% Triton-X-100, 0.1%acetylated BSA, pH 8.8. The incubation was carried out at 65 deg C. for20 minutes. After the incubation the wells were washed with Thermopol IIbuffer 3× and then subjected to cleavage with TCEP (100 mM) in 400 mMTris-HCl (pH=8.5) at 65 deg C. for 15 minutes. After the cleavage thewells were washed with the extension reaction buffer (3×) and subjectedto the next extension reaction. The wells were read out with finalreadout mixture consisting of: 2,3′-dideoxynucleotide mix (labeled)—2μM, Therminator II polymerase—250 μg/ml, 10 mM KCl, 10 mM (NH4)2SO₄, 20mM Tris-HCl, 2 mM MnSO₄, 0.1% Triton-X-10. The structures of thesenucleotides are presented in FIG. 23. After labeling cycle the slide waswashed with wash/block buffer (5×SSC, 0.1% Tween, 2% BSA), rinsed withwater and dried before imaging. Each well was imaged using a prototypesequencing instrument and bases were then called based on the relativeintensity of the observed signal. The result of the experiment ispresented in FIG. 24.

Example 10 Synthesis of 2′-Fluoro, 3′-O-Azidomethyl Nucleotides

The synthesis of 2′-fluoro-3′-O-azidomethyl-dNTPs is described in FIG.25. Briefly, reaction of 5′-O-TBDMS-2′-fluoro-2′-deoxynucleosides (1)with a mixture of DMSO, acetic acid, and acetic anhydride installed the3′-O-methylthiomethyl group (3′-O-MTM, 2), which upon treatment withSO₂Cl₂ converted to activated 3′-O—CH₂Cl (3). The2′-fluoro-3′-O—CH₂Cl₂′-deoxynucleoside (3) is then treated with NaN₃ indry DMF without purification to convert the 3′-O—CH₂Cl to 3′-O—CH₂N₃(4). 2′-fluoro-3′-O-azidomethyl-2′-deoxynucleosides of A, T, and C(5a-5c) can be obtained in good yield after deprotection of the5′-O-TBDMS group as described in FIG. 25. In case of2′-fluoro-3′-O-azidomethyl-2′-deoxybuanosine (G, 5d), the O⁶-group isprotected by diphenycarbamoyl group to increase yield. Finally, therespective nucleosides are phosphorylated using phosphorous oxychloridefollowed by tetrabutylammonium pyrophosphate in the presence of protonsponge (1,8-dimethylaminonaphthalene) and converted to their respectivetriphosphates (6).

Example 11 Synthesis of 2′-Fluoro, 3′-O-Azidomethyl PropargylaminoNucleotides

Synthesis of 2′-fluoro-3′-O-azidomethyl-(propargylamino)-dNTPs isdescribed in FIG. 26. Briefly, reaction of5′-O-TBDMS-2′-fluoro-(5/7-iodo*)-2′-deoxynucleosides (1) with a mixtureof N-trifluoroacetyl-propargylamine, tetrakis (triphenylphosphine)palladium (0) and CuI resulted in the formation of 5/7-propargylamidosubstituted nucleosides (2). In the next step the mixture of DMSO,acetic acid, and acetic anhydride installed the 3′-O-methylthiomethylgroup (3% O-MTM, 3), which upon treatment with SO₂Cl₂ converted toactivated 3′-O—CH₂Cl (4). The2′-fluoro-3′-O—CH₂Cl-5/7-propargylamido-2′-deoxynucleosides (4) werethen treated with NaN₃ in dry DMF without purification to convert the3′-O—CH₂Cl to 3′-O—CH₂N₃. (5)2′-fluoro-3′-O-azidomethyl-(propargylamino)-2′-deoxynucleosides of A, T,and C (5a-5c) can be obtained in good yield after deprotection of the5′-O-TBDMS group as described in FIG. 26. In case of2′-fluoro-3′-O-azidomethyl-2′-deoxybuanosine (G, 5d), the O⁶-group isprotected by diphenycarbamoyl group to increase yield. *5-iodo,2′-fluoro-2′-deoxy purines and 7-iodo-7-deaza-2′-fluoro-2′-deoxypyrimidines were used as starting material. The synthesis of thesecompounds is well known to those skilled in the art.

Example 12 Spectral Crosstalk Calibration

We calibrated the four color detection system of the above describedexemplary SBS device using a chip spotted with four separate dyes, onein each of four spots. We then made measurements of the chip in all fourchannels, calculated the spectral crosstalk factors and constructed theK and K⁻¹ matrices. FIG. 32 shows the effect of applying the spectralcrosstalk calibration matrix K−1 to raw sequencing data. The datademonstrates that the second base in the sequence would be miscalled asgreen were the spectral crosstalk calibration not performed.

Example 13 Re-Phasing Sequencing by Synthesis Data

As discussed above, dephasing of sequence data is cumulative and canpotentially be significant with longer read lengths. We applied thelead/lag compensation described above to both a 16-base and 25-basesequences containing an AGCT repeat. The results are shown in 33 and 34.FIG. 33A shows the original data captured by the fluorescent detectionsystem and FIG. 33B shows the data after being multiplied by thelead/lag compensation matrix with a lead parameter of 4.5% and a lagparameter of 1%. The relatively high lead parameter was probably due tonative nucleotide contamination in the polymerase preparation. FIG. 33illustrates how the compensation helps to correct miscalls. For example,bases at locations 11, 13 and 15 would be a miscall in the originaldephased data, but are correctly called (as are all the other bases) inthe rephased data. For the 25-base read in FIG. 34, the lead and lagparameters were 1.2% and 1.5% respectively. Although the lead and lagfor this sample were not large enough to create miscalls in the originaldata (FIG. 34A), the lead/lag correction does make the correct base astronger signal compared to the other colors (FIG. 34B). While in bothcorrected sequences (FIGS. 33B and 27B), the matrix multiplicationproduces some negative values, these are probably due to noise, and maybe ignored as long as they are small values. FIG. 34 shows that we wereable to generate data with high fidelity out to 25 bases.

Example 14 Sequencing by Synthesis Data Extra Washing

In this example, additional washing was done in an attempt to completelyremove the cleaving agent prior to the next cycle in sequencing bysynthesis. Interestingly, increased washing cycles after cleavage stephave only minimal effect on the sequencing performance, as illustratedin the Table below.

Rephased Data 25 nt 35 nt All Templates Templates Templates % CorrectWashes Lead Lag Lead Lag Calls Qa* 24 2.56% 1.69% 1.75% 1.12% 92.0%0.822 48 1.55% 2.32% 1.20% 1.80% 96.0% 0.862 100  1.40% 2.80% 0.95%1.65% 95.5% 0.826 *Qa = Intensity of the correct base signal/intensityof the second highest signalThe metric used to measure the dephasing process is the lead percentagederived empirically to compensate for the lead observed in the run. Onlyat very high wash cycles (i.e. too many washes to be practical) can oneimprove the base calling accuracy.

Example 15 Sequencing by Synthesis Data Using a Scavenger

In this example, scavengers were used in an attempt to inhibit anyremaining cleaving agent prior to the next cycle in sequencing bysynthesis. As noted above, such compounds can be included in thesolutions used for sequencing by synthesis (or in a separate additionalsolution if desired). In this example, the suitable operatingconcentration for the scavenger in the Extend A/B solutions wasexplored. Two different scavengers were used.

A. Cystamine Scavenger

3′-O-azidomethyl nucleotides labeled with dyes on a cleavable disulfidelinker were used. A range of scavenger concentrations were tested todetermine which concentration is acceptable by the polymerase. The tablebelow shows lead and lag values, and percentage of correct calls for the3′-O-azidomethy/disulfide chemistry in the absence and in the presenceof a first scavenger (cystamine @ 1 mM).

AVG Lead [%] AVG Lag [%] Correct calls [%] NO SCAVENGER 2.0 3.1 93.7CYSTAMINE 1.1 1.9 98.7 SCAVENGERIt is clear from the data in the table that the use of a scavenger canimprove the accuracy of base calling and reduce lead and lag.Importantly, extension reactions performed in the absence and in thepresence of this disulfide based scavenger, cystamine, showed theadditive does not significantly interfere with the extension reaction(FIG. 38).

B. ATA Scavenger

A second scavenger was also tested, i.e. the azido based scavenger, ATA:(11-Azido-3,6,9-trioxaundecan-1-amine). Extension reactions performed inthe absence and in the presence of this azido based scavenger.Nucleotides with 3′-O-azidomethyl groups and with azido based cleavablelinkers were used. The results (FIG. 39) show that the additive does notsignificantly interfere with the extension reaction.

Example 16 Synthesis of Disulfide-Dye Labeled 3′-O-AzidomethylNucleotide

In this example, a method is described for synthesizing a nucleotideanalogue containing an azidomethyl group on the 3″-OH and a labelattached via a disulfide linker (which is cleavable). The scheme isshown in FIG. 40. Preparation of the linker buffer solution: 11 mg of3-((2-aminoethyl)dithio)propionic acid hydrochloride (Prod #22101 fromPierce Biotech company, 2) was dissolved in 100 μl of 0.1 M sodiumbicarbonate and 900 μl of acetonitrile. 14 μl of triethylamine wasadded. To a solution of 6-carboxy-X-rhodamine, succinimidyl ester (6-ROXSE, cat. #C6126, Invitrogen, 1) (158 μl L, 25 mM, 3.96 mmol) in DMF wasadded the above linker solution (500 μl, 50 mM, 25.0 mmol). The reactionmixture was stirred overnight at room temperature and then 800 μl ofTEAB buffer (50 mM, pH 8) was added. The mixture was purified by HPLCand concentrated to give 0.51 μmol of product 3. HPLC method: A, 50 mMtriethylammonium bicarbonate (TEAB) buffer, pH=8.0; B, acetonitrile andeluted with a linear gradient of 0-70% B over 35 minutes and at a flowrate of 2 ml/min. The column used was NoaPak C18, 8×100 mm. Retentiontime for product is 20.5 min. Retention time for hydrolysis of startingmaterial is 18.2 min.

To the above linker-dye conjugate product 3 (0.51 μmol) in 300 μl of DMFwas added a solution of 2,6-dimethylaminopyridine (DMAP) (25 mM, 31 μl,0.77 μmol) and a solution of N,N′-disuccinimidyl carbonate (DSC) (25 mM,31 μl, 0.77 mmol). The reaction mixture was stirred for one hour at roomtemperature. 7-propargylamino, 3′-O-azidomethyl-dATP 5 (1.5 μmol) wasdissolved in 300 μl of water and 40 μl of tri-n-butylamine was added.All solvents were removed under vacuum and the residue dissolved in 300μl of DMF. This solution was then added to the activated linker-dyeconjugate 4 and the mixture was stirred overnight. The reaction mixturewas diluted with 800 μl of TEAB buffer (50 mM, pH 8), purified by HPLCand concentrated. 198 nmol of product 6 was obtained (Retention time forproduct is 18.5 min).

Example 17 Hot Embossing Millions to Billions of Beads on Slides orChips

In one embodiment, the present invention contemplates such microspheresor beads disposed at high density into microwells or indentations on asurface. It is not intended that the present invention be limited by thenature of the surface or the method of fabrication. Nonetheless, in oneembodiment, the present invention contemplates methods of fabrication togenerate beads on slides at high density.

In one preferred embodiment, the method relies on the use of a hotembossing technique as schematically shown in FIG. 41. Briefly, theprocess employs a stamp (80) having projections (81) that will createdesired features (83) of desired dimensions when pressed into thepolymer (82). The pressing step (B) is typically done with heat andpressure. Thereafter, the stamp is removed and the polymer is cooled(step C). Finally (step D), microspheres (84) containing biomolecules(85) are loaded into the microwells (86). In another embodiment, themethod relies on the use of injection molding technique.

It is not intended that the present invention be limited by the natureof the polymer used in performing the hot embossing or molding process.A variety of polymers can be used including but not limited to: PMMA(polymethyl methacrylate), COP (cycloolefine polymer), and COC(cycloolefine copolymer). In the case of polymers that lack naturalfunctional groups on the surface these groups can be grafted on thesurface by performing ozonation, oxidation, corona discharge treatment,surface plasma or UV treatment or combination thereof. These fabricationmethods allow one to generate substrates with varying features/wellsdensity. Using standard size microscope slides casted out of PMMA or COPpolymers one can create wells with 20 um, 5 um, and 1 um diameters. Theslides with approximately 5 um features (e.g. between 4.8 um and 5.3 um)contain about 40 million microwells per slide, while the 1 um featureslide contains about 1 billion features per slide. With thebiomolecule-containing microspheres deposited within the microwells, asingle slide with such features permits a variety of high throughput,robust assays (e.g. sequencing by synthesis, hybridization, etc.).Nucleic acid fragments representing a large portion of a genome (e.g.human genome) or even an entire genome can be placed on a single slideor handful of slides, and then assayed sequentially or simultaneously.

Example 18 Sequencing Changing the Spacer Arm Groups or Charge

When performing sequencing by synthesis process one needs to use labelednucleotides to be able to read the signal. In most cases these labelednucleotides after cleavage result in structures that is not of thenative nucleotide. For example, if one uses only labeled nucleotides theDNA structure after cleavage of the dye looks like one shown in FIG. 42(right side). As can be seen, the spacer arm used to attach the dye tothe base still remains attached.

In some cases the spacer also carries a charge, such as for example whenpropargylamino nucleotides are used. In the case of disulfide bonds whatremains after cleavage is the spacer arm with thiol (SH) group attached.The presence of these spacers and groups may affect the ability of thesequencing polymerases to incorporate the subsequent nucleotide. Oneapproach to minimize or eliminate this undesirable effect is to changethe reactivity of the spacer arm groups or their charge by performing achemical “capping” step, where specific reagent is added to react onlywith groups on the spacer arm. This is shown schematically in FIG. 43.

Example 19 Sequencing by Synthesis Data Using Labeled and UnlabeledNucleotides

As noted previously, the presence of the linkers, spacers and groups onnucleotides may affect the ability of the sequencing polymerases toincorporate the subsequent nucleotide. One approach to minimize oreliminate this undesirable effect is to reduce the amount of labelednucleotides incorporated in the template. Reducing the amount of labelednucleotides that are incorporated can be accomplished by reducing theconcentration of labeled nucleotides in the extension solution, and/ormixing labeled nucleotides (reversible terminators) with non-labeledreversibly terminating nucleotides. In contrast to labeled nucleotides,non-labeled reversible terminator nucleotides after cleavage convert tonative nucleotide.

The effect of reducing the concentration of labeled nucleotides can bebest observed by measuring the ability of polymerase to incorporate thesubsequent nucleotides efficiently and with high fidelity. This is shownin FIGS. 44, 45 and 46.

When the amount of labeled nucleotides is reduced, this results inreduction of fluorescent signal as shown in FIG. 47 (where only labelednucleotides are used in successive extention reactions). In principleonly the amount of signal necessary to decode the nucleotide isrequired. In addition to changing the ratio of labeled and unlabelednucleotides and optimizing it for particular polymerase, one can alsoadjust the time of extension (e.g. reduce extension times down to 1-2minutes) to gain even better control on the signal/incorporation ratioof labeled nucleotides. This is shown in FIGS. 48 and 49 whereadditional performance improvement is achieved upon reducing extensiontime (to 2 minutes and 1 minute, respectively).

All publications and patents mentioned in the above specification areherein incorporated by reference. Various modifications and variationsof the described methods and system of the invention will be apparent tothose skilled in the art without departing from the scope and spirit ofthe invention. Although the invention has been described in connectionwith specific embodiments, it should be understood that the invention asclaimed should not be unduly limited to such specific embodiments.Indeed, various modifications of the described modes for carrying outthe invention which are obvious to those skilled in the art and infields related thereto are intended to be within the scope of thefollowing claims.

1. A method of incorporating labeled nucleotides into nucleic acid,comprising: a) providing a plurality of nucleic acid template molecules,a polymerase, and a plurality of nucleotide analogues wherein eachnucleotide analogue is labeled with a unique label and contains aremovable chemical moiety capping the 3′-OH group, wherein the label isattached to the base through a hydroxymethyl group; b) incorporating afirst nucleotide analogue with said polymerase; and c) detecting thelabel of the incorporated nucleotide analogue.